Found a subtle bug caused by an implicit enum-to-bool conversion (of the TentativePar...

Found a subtle bug caused by an implicit enum-to-bool conversion (of the TentativeParsingResult enum).
This was the motivation of the following changes:

-'TentativeParsingResult' enum is replaced by a 'TPResult' class that basically encapsulates the enum.
-TPR_true, TPR_false, TPR_ambiguous, and TPR_error enum constants are replaced by TPResult::True(), TPResult::False(), etc. calls that return a TPResult object.
-Also fixed the subtle bug in Parser::isCXXFunctionDeclarator (caught by the above changes as a compilation error).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57125 91177308-0d34-0410-b5e6-96231b3b80d8