SerializeLoc: serialize basic decl source location information to .swiftsourceinfo file

After setting up the .swiftsourceinfo file, this patch starts to actually serialize
and de-serialize source locations for declaration. The binary format of .swiftsourceinfo
currently contains these three records:

BasicDeclLocs: a hash table mapping from a USR ID to a list of basic source locations. The USR id
could be retrieved from the following DeclUSRs record using an actual decl USR. The basic source locations
include a file ID and the results from Decl::getLoc(), ValueDecl::getNameLoc(), Decl::getStartLoc() and Decl::getEndLoc().
The file ID could be used to retrieve the actual file name from the following SourceFilePaths record.
Each location is encoded as a line:column pair.

DeclUSRS: a hash table mapping from USR to a USR ID used by location records.

SourceFilePaths: a hash table mapping from a file ID to actual file name.

BasicDeclLocs should be sufficient for most diagnostic cases. If additional source locations
are needed, we could always add new source location records without breaking the backward compatibility.
When de-serializing the source location from a module-imported decl, we calculate its USR, retrieve the USR ID
from the DeclUSRS record, and use the USR ID to look up the basic location list in the BasicDeclLocs record.

For more details about .swiftsourceinfo file: https://forums.swift.org/t/proposal-emitting-source-information-file-during-compilation

include/swift/Serialization/SerializedModuleLoader.h

@@ -53,6 +53,7 @@ protected:
   bool findModule(AccessPathElem moduleID,
                   std::unique_ptr<llvm::MemoryBuffer> *moduleBuffer,
                   std::unique_ptr<llvm::MemoryBuffer> *moduleDocBuffer,
+                  std::unique_ptr<llvm::MemoryBuffer> *moduleSourceInfoBuffer,
                   bool &isFramework, bool &isSystemModule);
 
   /// Attempts to search the provided directory for a loadable serialized
@@ -70,20 +71,30 @@ protected:
   virtual std::error_code findModuleFilesInDirectory(
       AccessPathElem ModuleID, StringRef DirPath, StringRef ModuleFilename,
       StringRef ModuleDocFilename,
+      StringRef ModuleSourceInfoFilename,
       std::unique_ptr<llvm::MemoryBuffer> *ModuleBuffer,
-      std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer) = 0;
+      std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer,
+      std::unique_ptr<llvm::MemoryBuffer> *ModuleSourceInfoBuffer) = 0;
 
   std::error_code
   openModuleFiles(AccessPathElem ModuleID,
                   StringRef ModulePath, StringRef ModuleDocPath,
+                  StringRef ModuleSourceInfoPath,
                   std::unique_ptr<llvm::MemoryBuffer> *ModuleBuffer,
-                  std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer);
+                  std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer,
+                  std::unique_ptr<llvm::MemoryBuffer> *ModuleSourceInfoBuffer);
 
   std::error_code
   openModuleDocFile(AccessPathElem ModuleID,
                     StringRef ModuleDocPath,
                     std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer);
 
+  std::error_code
+  openModuleSourceInfoFile(AccessPathElem ModuleID,
+                           StringRef ModulePath,
+                           StringRef ModuleSourceInfoPath,
+                           std::unique_ptr<llvm::MemoryBuffer> *ModuleSourceInfoBuffer);
+
   /// If the module loader subclass knows that all options have been tried for
   /// loading an architecture-specific file out of a swiftmodule bundle, try
   /// to list the architectures that \e are present.
@@ -116,6 +127,7 @@ public:
   FileUnit *loadAST(ModuleDecl &M, Optional<SourceLoc> diagLoc,
                     std::unique_ptr<llvm::MemoryBuffer> moduleInputBuffer,
                     std::unique_ptr<llvm::MemoryBuffer> moduleDocInputBuffer,
+                    std::unique_ptr<llvm::MemoryBuffer> moduleSourceInfoInputBuffer,
                     bool isFramework, bool treatAsPartialModule);
 
   /// Check whether the module with a given name can be imported without
@@ -163,8 +175,10 @@ class SerializedModuleLoader : public SerializedModuleLoaderBase {
   std::error_code findModuleFilesInDirectory(
       AccessPathElem ModuleID, StringRef DirPath, StringRef ModuleFilename,
       StringRef ModuleDocFilename,
+      StringRef ModuleSourceInfoFilename,
       std::unique_ptr<llvm::MemoryBuffer> *ModuleBuffer,
-      std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer) override;
+      std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer,
+      std::unique_ptr<llvm::MemoryBuffer> *ModuleSourceInfoBuffer) override;
 
   bool maybeDiagnoseTargetMismatch(SourceLoc sourceLocation,
                                    StringRef moduleName,
@@ -203,8 +217,10 @@ class MemoryBufferSerializedModuleLoader : public SerializedModuleLoaderBase {
   std::error_code findModuleFilesInDirectory(
       AccessPathElem ModuleID, StringRef DirPath, StringRef ModuleFilename,
       StringRef ModuleDocFilename,
+      StringRef ModuleSourceInfoFilename,
       std::unique_ptr<llvm::MemoryBuffer> *ModuleBuffer,
-      std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer) override;
+      std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer,
+      std::unique_ptr<llvm::MemoryBuffer> *ModuleSourceInfoBuffer) override;
 
   bool maybeDiagnoseTargetMismatch(SourceLoc sourceLocation,
                                    StringRef moduleName,
@@ -317,6 +333,8 @@ public:
 
   Optional<StringRef> getGroupNameByUSR(StringRef USR) const override;
 
+  Optional<BasicDeclLocs> getBasicLocsForDecl(const Decl *D) const override;
+
   void collectAllGroups(std::vector<StringRef> &Names) const override;
 
   virtual void getTopLevelDecls(SmallVectorImpl<Decl*> &results) const override;