Fix use-after-destruction introduced in r224924.

getMainExecutable() returns a std::string, assigning its result
to StringRef immediately creates a dangling pointer. This was
detected by half-broken fast-MSan-bootstrap bot.

llvm-svn: 224956

clang/lib/Frontend/CompilerInvocation.cpp

@@ -983,7 +983,8 @@ static InputKind ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
 
 std::string CompilerInvocation::GetResourcesPath(const char *Argv0,
                                                  void *MainAddr) {
-  StringRef ClangExecutable = llvm::sys::fs::getMainExecutable(Argv0, MainAddr);
+  std::string ClangExecutable =
+      llvm::sys::fs::getMainExecutable(Argv0, MainAddr);
   StringRef Dir = llvm::sys::path::parent_path(ClangExecutable);
 
   // Compute the path to the resource directory.