[llvm] [Support] Fix memory leak induced by `sys::RemoveFileOnSignal` (PR #159984)

[via llvm-commits]

================
@@ -421,8 +421,13 @@ bool sys::RemoveFileOnSignal(StringRef Filename, std::string *ErrMsg) {
     return true;
   }
 
-  if (FilesToRemove == NULL)
+  if (FilesToRemove == NULL) {
----------------
yronglin wrote:

I think we can change `FilesToRemove` from `static std::vector<std::string> *FilesToRemove = NULL;` to `static std::vector<std::string> FilesToRemove;`, and change all pices where `FilesToRemove == NULL` to `FilesToRemove.empty()`, What do you think?

https://github.com/llvm/llvm-project/pull/159984