Add SearchPathRemapper to SearchPathOptions

This is intended to be used from LLDB to apply the remappings
specified in target.source-map to remap any serialized
Swiftmodule search paths that were prefixed using
`-prefix-serialized-debugging-options`.

Author: rmaz

include/swift/AST/SearchPathOptions.h

@@ -14,6 +14,7 @@
 #define SWIFT_AST_SEARCHPATHOPTIONS_H
 
 #include "swift/Basic/ArrayRefView.h"
+#include "swift/Basic/PathRemapper.h"
 #include "llvm/ADT/Hashing.h"
 
 #include <string>
@@ -97,6 +98,11 @@ class SearchPathOptions {
 
   /// A file containing modules we should perform batch scanning.
   std::string BatchScanInputFilePath;
+
+  /// Debug path mappings to apply to serialized search paths. These are
+  /// specified in LLDB from the target.source-map entries.
+  PathRemapper SearchPathRemapper;
+
 private:
   static StringRef
   pathStringFromFrameworkSearchPath(const FrameworkSearchPath &next) {

lib/Serialization/ModuleFile.cpp

@@ -158,9 +158,11 @@ Status ModuleFile::associateWithFileContext(FileUnit *file, SourceLoc diagLoc,
     return error(status);
   }
 
-  for (const auto &searchPath : Core->SearchPaths)
-    ctx.addSearchPath(searchPath.Path, searchPath.IsFramework,
-                      searchPath.IsSystem);
+  for (const auto &searchPath : Core->SearchPaths) {
+    ctx.addSearchPath(
+        ctx.SearchPathOpts.SearchPathRemapper.remapPath(searchPath.Path),
+        searchPath.IsFramework, searchPath.IsSystem);
+  }
 
   auto clangImporter = static_cast<ClangImporter *>(ctx.getClangModuleLoader());