[AST] Compress the FixedPointSemantics type better.

[ebevhan]

Rebased.

[rjmccall]

Thanks, LGTM.

[ebevhan]

Rebased.

clang/include/clang/Basic/FixedPoint.h

public:
   static FixedPointSemantics GetIntegerSemantics(unsigned Width,
                                                  bool IsSigned) {
     return FixedPointSemantics(Width, /*Scale=*/0, IsSigned,
                                /*IsSaturated=*/false,
                                /*HasUnsignedPadding=*/false);
   }
 
 private:
-  unsigned Width;
-  unsigned Scale;
-  bool IsSigned;
-  bool IsSaturated;
-  bool HasUnsignedPadding;
+  unsigned Width          : 16;
+  unsigned Scale          : 13;
+  unsigned IsSigned       : 1;
+  unsigned IsSaturated    : 1;
+  unsigned HasUnsignedPadding : 1;
 };
 
 /// The APFixedPoint class works similarly to APInt/APSInt in that it is a
 /// functional replacement for a scaled integer. It is meant to replicate the
 /// fixed point types proposed in ISO/IEC JTC1 SC22 WG14 N1169. The class carries
 /// info about the fixed point type's width, sign, scale, and saturation, and
 /// provides different operations that would normally be performed on fixed point
 /// types.