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.
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.