Add a -debug-module-path frontend option

This new option allows the Driver to pass the path to a compilation
job's own binary swiftmodule artifact to the frontend. The compiler
then stores this path in the debug info, to allow clients like LLDB to
unambiguously know which binary Swift module belongs to which compile
unit.

rdar://163302154

include/swift/AST/IRGenOptions.h

@@ -270,6 +270,9 @@ class IRGenOptions {
 public:
   std::string ModuleName;
 
+  /// The path to the main binary swiftmodule for the debug info.
+  std::string DebugModulePath;
+
   /// The compilation directory for the debug info.
   std::string DebugCompilationDir;
 

include/swift/Option/Options.td

@@ -643,6 +643,12 @@ def project_name : Separate<["-"], "project-name">,
 def module_name_EQ : Joined<["-"], "module-name=">, Flags<[FrontendOption]>,
   Alias<module_name>;
 
+def debug_module_path : Separate<["-"], "debug-module-path">,
+  Flags<[FrontendOption]>,
+  HelpText<"Path to this module's binary swiftmodule artifact (required by debug info)">;
+def debug_module_path_EQ : Joined<["-"], "debug-module-path=">, Flags<[FrontendOption]>,
+  Alias<debug_module_path>;
+
 def module_alias : Separate<["-"], "module-alias">,
   Flags<[FrontendOption, ModuleInterfaceOption]>,
   MetaVarName<"<alias_name=real_name>">,

lib/Frontend/CompilerInvocation.cpp

@@ -3460,6 +3460,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
                      A->getAsString(Args), A->getValue());
   }
 
+  if (const Arg *A = Args.getLastArg(options::OPT_debug_module_path))
+    Opts.DebugModulePath = A->getValue();
+  
   for (auto A : Args.getAllArgValues(options::OPT_file_prefix_map)) {
     auto SplitMap = StringRef(A).split('=');
     Opts.FilePrefixMap.addMapping(SplitMap.first, SplitMap.second);

lib/IRGen/IRGenDebugInfo.cpp

@@ -864,7 +864,7 @@ private:
   llvm::DIModule *getOrCreateModule(const void *Key, llvm::DIScope *Parent,
                                     StringRef Name, StringRef IncludePath,
                                     uint64_t Signature = ~1ULL,
-                                    StringRef ASTFile = StringRef()) {
+                                    StringRef ASTFile = {}) {
     // Look in the cache first.
     auto Val = DIModuleCache.find(Key);
     if (Val != DIModuleCache.end())
@@ -2823,8 +2823,12 @@ IRGenDebugInfoImpl::IRGenDebugInfoImpl(const IRGenOptions &Opts,
 
   // Create a module for the current compile unit.
   auto *MDecl = IGM.getSwiftModule();
-  llvm::sys::path::remove_filename(SourcePath);
-  MainModule = getOrCreateModule(MDecl, TheCU, Opts.ModuleName, SourcePath);
+  StringRef Path = Opts.DebugModulePath;
+  if (Path.empty()) {
+    llvm::sys::path::remove_filename(SourcePath);
+    Path = SourcePath;
+  }
+  MainModule = getOrCreateModule(MDecl, TheCU, Opts.ModuleName, Path);
   DBuilder.createImportedModule(MainFile, MainModule, MainFile, 0);
 
   // Macro definitions that were defined by the user with "-Xcc -D" on the

test/DebugInfo/ebm_module_path.swift

@@ -0,0 +1,4 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-ir -g -module-name=A -debug-module-path %t/MY_MODULE_PATH.swiftmodule -emit-ir -o - | %FileCheck %s
+
+// CHECK: DIModule(scope: null, name: "A", includePath: "{{.*}}MY_MODULE_PATH.swiftmodule")
+