[sourcekit] Fix use-after-free of request string

The CursorInfoConsumer outlives the request call, so we need to copy the
input filename string. This fixes a use-after-free elucidated by asan.

Author: benlangmuir

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -1162,7 +1162,7 @@ class CursorRangeInfoConsumer : public SwiftASTConsumer {
 protected:
   SwiftLangSupport ⟪
   SwiftInvocationRef ASTInvok;
-  StringRef InputFile;
+  std::string InputFile;
   unsigned Offset;
   unsigned Length;
 
@@ -1181,7 +1181,7 @@ class CursorRangeInfoConsumer : public SwiftASTConsumer {
   CursorRangeInfoConsumer(StringRef InputFile, unsigned Offset, unsigned Length,
                           SwiftLangSupport &Lang, SwiftInvocationRef ASTInvok,
                           bool TryExistingAST, bool CancelOnSubsequentRequest)
-    : Lang(Lang), ASTInvok(ASTInvok),InputFile(InputFile), Offset(Offset),
+    : Lang(Lang), ASTInvok(ASTInvok),InputFile(InputFile.str()), Offset(Offset),
       Length(Length), TryExistingAST(TryExistingAST),
       CancelOnSubsequentRequest(CancelOnSubsequentRequest) {}