Fix an incorrect bit-width for storing attribute syntax information

This field corresponds to the Syntax enumeration, and that gained
another entry in 1fdf952. However,
the bit-field for storing the syntax used was not adjusted to handle
the extra field.

This turns out to be unobservable for HLSL attributes at the moment, so
there is no test coverage. But it's also not really an NFC change
either.

clang/include/clang/Basic/AttributeCommonInfo.h

@@ -69,7 +69,7 @@ class AttributeCommonInfo {
   // Corresponds to the Kind enum.
   unsigned AttrKind : 16;
   /// Corresponds to the Syntax enum.
-  unsigned SyntaxUsed : 3;
+  unsigned SyntaxUsed : 4;
   unsigned SpellingIndex : 4;
 
 protected: