Add a comment that describes tryEvaluate.  Make tryEvaluate fold
__builtin_constant_p properly, and add some scaffolding for
FloatExprEvaluator to eventually handle huge_val and inf.

llvm-svn: 57152
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 562cf12..bd1578f 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -171,7 +171,7 @@
   if (isBuiltinCall() != Builtin::BI__builtin_classify_type)
     return false;
   
-  // The following enum mimics gcc's internal "typeclass.h" file.
+  // The following enum mimics the values returned by GCC.
   enum gcc_type_class {
     no_type_class = -1,
     void_type_class, integer_type_class, char_type_class,