Fix LLVM_ENABLE_THREADS check from 26a92d5852b2c6bf77efd26f6c0194c913f40285

[arichardson]

Summary

We should be using #if instead of #ifdef here since LLVM_ENABLE_THREADS
is set using #cmakedefine01 so is always defined.

Since the other branch has never been used I wonder if we should just remove it instead?

[aaron.ballman]

LGTM!

Since the other branch has never been used I wonder if we should just remove it instead?

I don't think removing the other branch entirely is a good approach because then the call will silently be a noop when threads are disabled. I'd rather we keep the support, or turn the other branch into a loud breakage so that callers of it find out at compile time that it's not supported when threads are disabled.

clang/include/clang/Basic/Stack.h

namespace clang {
 
   void runWithSufficientStackSpaceSlow(llvm::function_ref<void()> Diag,
                                        llvm::function_ref<void()> Fn);
 
   /// Run a given function on a stack with "sufficient" space. If stack space
   /// is insufficient, calls Diag to emit a diagnostic before calling Fn.
   inline void runWithSufficientStackSpace(llvm::function_ref<void()> Diag,
                                           llvm::function_ref<void()> Fn) {
-#ifdef LLVM_ENABLE_THREADS
+#if LLVM_ENABLE_THREADS
     if (LLVM_UNLIKELY(isStackNearlyExhausted()))
       runWithSufficientStackSpaceSlow(Diag, Fn);
     else
       Fn();
 #else
     if (LLVM_UNLIKELY(isStackNearlyExhausted()))
       Diag();
     Fn();
 #endif
   }
 } // end namespace clang
 
 #endif // LLVM_CLANG_BASIC_STACK_H