[Parser][ObjC++] Improve diagnostics and recovery when C++ keywords are used

as identifiers in Objective-C++

This commit improves the 'expected identifier' errors that are presented when a
C++ keyword is used as an identifier in Objective-C++ by mentioning that this is
a C++ keyword in the diagnostic message. It also improves the error recovery:
the parser will now treat the C++ keywords as identifiers to prevent unrelated
parsing errors.

rdar://20626062

Differential Revision: https://reviews.llvm.org/D26503

llvm-svn: 299950