[Support][Windows] Use the original path if GetFinalPathNameByHandleW() failed

The commit f11b056 (llvm#76304) breaks `clang` and other tools if they are used from a RAMDrive.
`GetFinalPathNameByHandleW()` may return 0 and GetLastError 0x28. This patch fixes that issue.
Note `real_path()` uses `openFileForRead()` but it reports the error only if failed to open a file. Getting `RealPath` is optional functionality.

BTW, `sys::fs::real_path()` resolves not only symlinks, but also network drives and virtual drives created by the `subst` tool. It may break an automation. It is better to detect symlinks and resolve only symlinks.

Author: slydiman

llvm/lib/Support/Windows/Path.inc

@@ -157,7 +157,9 @@ std::string getMainExecutable(const char *argv0, void *MainExecAddr) {
 
   SmallString<256> RealPath;
   sys::fs::real_path(PathNameUTF8, RealPath);
-  return std::string(RealPath);
+  if (RealPath.size())
+    return std::string(RealPath);
+  return std::string(PathNameUTF8.data());
 }
 
 UniqueID file_status::getUniqueID() const {