Revert "[clang] Remove #undef alloca workaround (#81534)"

This reverts commit 742a06f.

Author: Prabhuk

clang/include/clang/Basic/Builtins.h

@@ -20,6 +20,10 @@
 #include "llvm/ADT/StringRef.h"
 #include <cstring>
 
+// VC++ defines 'alloca' as an object-like macro, which interferes with our
+// builtins.
+#undef alloca
+
 namespace clang {
 class TargetInfo;
 class IdentifierTable;