[Support] Don't modify the current EH context during stack unwinding

Copy it instead. Otherwise, key registers (such as RBP) may get zeroed
out by the stack unwinder.

Fixes CrashRecoveryTest.DumpStackCleanup with MSVC in release builds.

Reviewed By: stella.stamenova

Differential Revision: https://reviews.llvm.org/D73809

(cherry picked from commit b074acb)

llvm/lib/Support/Windows/Signals.inc

@@ -820,7 +820,13 @@ static LONG WINAPI LLVMUnhandledExceptionFilter(LPEXCEPTION_POINTERS ep) {
                    << "\n";
   }
 
-  LocalPrintStackTrace(llvm::errs(), ep ? ep->ContextRecord : nullptr);
+  // Stack unwinding appears to modify the context. Copy it to preserve the
+  // caller's context.
+  CONTEXT ContextCopy;
+  if (ep)
+    memcpy(&ContextCopy, ep->ContextRecord, sizeof(ContextCopy));
+
+  LocalPrintStackTrace(llvm::errs(), ep ? &ContextCopy : nullptr);
 
   return EXCEPTION_EXECUTE_HANDLER;
 }