Fix sourcekitd persistent file-locks on Windows

Author: z2oh

include/swift/Basic/SourceManager.h

@@ -94,6 +94,7 @@ class SourceManager {
 private:
   llvm::SourceMgr LLVMSourceMgr;
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FileSystem;
+  bool EditorMode = false;
   unsigned IDEInspectionTargetBufferID = 0U;
   unsigned IDEInspectionTargetOffset;
 
@@ -166,6 +167,10 @@ class SourceManager {
     return FileSystem;
   }
 
+  void setEditorMode() {
+    EditorMode = true;
+  }
+
   void setIDEInspectionTarget(unsigned BufferID, unsigned Offset) {
     assert(BufferID != 0U && "Buffer should be valid");
 

lib/Basic/SourceLoc.cpp

@@ -695,7 +695,11 @@ unsigned SourceManager::getExternalSourceBufferID(StringRef Path) {
     return It->getSecond();
   }
   unsigned Id = 0u;
-  auto InputFileOrErr = swift::vfs::getFileOrSTDIN(*getFileSystem(), Path);
+  auto InputFileOrErr =
+      swift::vfs::getFileOrSTDIN(*getFileSystem(), Path,
+                                 /* FileSize */ -1,
+                                 /* RequiresNullTerminator */ true,
+                                 /* isVolatile */ this->EditorMode);
   if (InputFileOrErr) {
     // This assertion ensures we can look up from the map in the future when
     // using the same Path.

lib/Frontend/Frontend.cpp

@@ -552,6 +552,12 @@ bool CompilerInstance::setup(const CompilerInvocation &Invoke,
   if (LangOpts.EnableModuleLoadingRemarks) {
     Invocation.getSearchPathOptions().dump(LangOpts.Target.isOSDarwin());
   }
+  
+  // If the compiler instance is managed by an IDE, inform the source manager to avoid
+  // memory-mapping source files that are likely to be mutated in an editor.
+  if (LangOpts.DiagnosticsEditorMode) {
+    this->getSourceMgr().setEditorMode();
+  }
 
   // If we expect an implicit stdlib import, load in the standard library. If we
   // either fail to find it or encounter an error while loading it, bail early. Continuing will at best