[PATCH] [YAML] Add an optional argument `EnumMask` to the `yaml::IO::bitSetCase()`

[kledzik at apple.com]

Please update the yaml documentation (llvm/docs/YamlIO.rst) with this new functionality too.

BTW, I had this same issue with mach-o in which the uint32_t section "flags" field has a 8-bit enum and 24-bits of flags.  What I did was break them into two yaml fields.

================
Comment at: include/llvm/Support/YAMLTraits.h:475-483
@@ -474,8 +474,11 @@
 
   template <typename T>
-  void bitSetCase(T &Val, const char* Str, const T ConstVal) {
-    if ( bitSetMatch(Str, outputting() && (Val & ConstVal) == ConstVal) ) {
+  void bitSetCase(T &Val, const char *Str, const T ConstVal,
+                  const T EnumMask = T(0)) {
+    bool Match = (EnumMask && (Val & EnumMask) == ConstVal) ||
+                 (!EnumMask && (Val & ConstVal) == ConstVal);
+    if (bitSetMatch(Str, outputting() && Match)) {
       Val = Val | ConstVal;
     }
   }
 
----------------
I think it would be cleaner to add a new method:
    void maskedBitSetCase()
or 
    void bitSetCaseWithMask()
rather that adding that optional parameter, because now you have to check if the mask is zero (not used) and use different bit checking.

http://reviews.llvm.org/D3807