Preprocessor: don't exit early in CheckMacroName()

The checks below can hypothetically apply to converted operator name
identifiers.

In practice there are no builtin macros etc. with those names so there's no
behavioural change to test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209962 91177308-0d34-0410-b5e6-96231b3b80d8
Preprocessor: recover gracefully when C++ operator names are used as macro identifiers

This failure mode shows up occasionally when users try to include C headers in
C++ projects or when porting from Windows. We might as well recover in the way
the user expected, thus avoiding confusing diagnostic messages at point of use.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209963 91177308-0d34-0410-b5e6-96231b3b80d8