[ASTMatchers] Fixed CastKind being parsed incorrectly for dynamic matchers

[njames93]

Summary

Requires hasCastKind arguments to have CK_ prefixed to bring it in line with the documentation and other matchers that take enumerations.

[njames93]

• Added support for giving suggestions on invalid inputs

[njames93]

Fixed up documentation for CastKind matcher.

[aaron.ballman]

Can you also add some unit tests to the dynamic matcher tests?

[njames93]

Added unit test for matcher

[aaron.ballman]

LGTM!

clang/lib/ASTMatchers/Dynamic/Marshallers.h

static ArgKind getKind() {
     return ArgKind(ArgKind::AK_String);
   }
 };
 
 template <> struct ArgTypeTraits<CastKind> {
 private:
   static Optional<CastKind> getCastKind(llvm::StringRef AttrKind) {
     return llvm::StringSwitch<Optional<CastKind>>(AttrKind)
-#define CAST_OPERATION(Name) .Case( #Name, CK_##Name)
+#define CAST_OPERATION(Name) .Case("CK_" #Name, CK_##Name)
 #include "clang/AST/OperationKinds.def"
         .Default(llvm::None);
   }
 
 public:
   static bool is(const VariantValue &Value) {
     return Value.isString() && getCastKind(Value.getString());
   }