[Support] Fix Windows dump file hang with multi-threaded crashes

Prevents deadlock between MiniDumpWriteDump and
CryptAcquireContextW (called via fs::createTemporaryFile) in
WriteWindowsDumpFile.

However, there's no guarantee that deadlock can't still occur between
MiniDumpWriteDump and some other Win32 API call. But that would appear
to be the "accepted" risk of using MiniDumpWriteDump in this manner.

Differential Revision: https://reviews.llvm.org/D129004

llvm/lib/Support/Windows/Signals.inc

@@ -731,6 +731,11 @@ static bool GetDumpType(HKEY Key, MINIDUMP_TYPE &ResultType) {
 /// otherwise.
 static std::error_code WINAPI
 WriteWindowsDumpFile(PMINIDUMP_EXCEPTION_INFORMATION ExceptionInfo) {
+  struct ScopedCriticalSection {
+    ScopedCriticalSection() { EnterCriticalSection(&CriticalSection); }
+    ~ScopedCriticalSection() { LeaveCriticalSection(&CriticalSection); }
+  } SCS;
+
   using namespace llvm;
   using namespace llvm::sys;