Add LLVM_ATTRIBUTE_UNINITIALIZED

Add LLVM_ATTRIBUTE_UNINITIALIZED to map to __attribute__((uninitialized)) when
available.

Author: Cameron McInally

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -3177,7 +3177,7 @@ StringRef tools::ParseMPreferVectorWidthOption(clang::DiagnosticsEngine &Diags,
     return "";
 
   StringRef Value = A->getValue();
-  unsigned Width;
+  unsigned Width LLVM_ATTRIBUTE_UNINITIALIZED;
 
   // Only "none" and Integer values are accepted by
   // -mprefer-vector-width=<value>.

llvm/include/llvm/Support/Compiler.h

@@ -238,6 +238,12 @@
 #define LLVM_PREFETCH(addr, rw, locality)
 #endif
 
+#if __has_attribute(uninitialized)
+#define LLVM_ATTRIBUTE_UNINITIALIED __attribute__((uninitialized))
+#else
+#define LLVM_ATTRIBUTE_UNINITIALIZED
+#endif
+
 #if __has_attribute(used)
 #define LLVM_ATTRIBUTE_USED __attribute__((__used__))
 #else