Make the C++ exceptions here consistent

Adding a C source file somehow exposed an issue in an unrelated C++ file.
Thanks to Joe Groff for the fix.

Author: tbkka

stdlib/public/runtime/WeakReference.h

@@ -113,7 +113,7 @@ class WeakReferenceBits {
 
  public:
   LLVM_ATTRIBUTE_ALWAYS_INLINE
-  WeakReferenceBits() { }
+  WeakReferenceBits() noexcept { }
 
   LLVM_ATTRIBUTE_ALWAYS_INLINE
   WeakReferenceBits(HeapObjectSideTableEntry *newValue) {