[Dependency Scanning] Remove obsolete placeholder module concept

This was used a long time ago for a design of a scanner which could rely on the client to specify that some modules *will be* present at a given location but are not yet during the scan. We have long ago determined that the scanner must have all modules available to it at the time of scan for soundness. This code has been stale for a couple of years and it is time to simplify things a bit by deleting it.

include/swift/DependencyScan/SerializedModuleDependencyCacheFormat.h

@@ -39,7 +39,7 @@ using llvm::BCVBR;
 
 /// Every .moddepcache file begins with these 4 bytes, for easy identification.
 const unsigned char MODULE_DEPENDENCY_CACHE_FORMAT_SIGNATURE[] = {'I', 'M', 'D','C'};
-const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MAJOR = 9;
+const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MAJOR = 10;
 /// Increment this on every change.
 const unsigned MODULE_DEPENDENCY_CACHE_FORMAT_VERSION_MINOR = 3;
 
@@ -109,7 +109,6 @@ enum {
   OPTIONAL_IMPORT_STATEMENT_ARRAY_NODE,
   SWIFT_INTERFACE_MODULE_DETAILS_NODE,
   SWIFT_SOURCE_MODULE_DETAILS_NODE,
-  SWIFT_PLACEHOLDER_MODULE_DETAILS_NODE,
   SWIFT_BINARY_MODULE_DETAILS_NODE,
   CLANG_MODULE_DETAILS_NODE,
   IDENTIFIER_NODE,
@@ -204,7 +203,6 @@ using OptionalImportStatementArrayLayout =
 // - SwiftInterfaceModuleDetails
 // - SwiftSourceModuleDetails
 // - SwiftBinaryModuleDetails
-// - SwiftPlaceholderModuleDetails
 // - ClangModuleDetails
 using ModuleInfoLayout =
     BCRecordLayout<MODULE_NODE,                    // ID
@@ -269,13 +267,6 @@ using SwiftBinaryModuleDetailsLayout =
                    IdentifierIDField                 // UserModuleVersion
                    >;
 
-using SwiftPlaceholderModuleDetailsLayout =
-    BCRecordLayout<SWIFT_PLACEHOLDER_MODULE_DETAILS_NODE, // ID
-                   FileIDField,                           // compiledModulePath
-                   FileIDField,                           // moduleDocPath
-                   FileIDField                            // moduleSourceInfoPath
-                   >;
-
 using ClangModuleDetailsLayout =
     BCRecordLayout<CLANG_MODULE_DETAILS_NODE, // ID
                    FileIDField,               // pcmOutputPath