CheckC*CompilerFlag: add documentation what to expect from a positive result
CheckC*CompilerFlags: move common patterns to their own file
Merge topic 'cxx-flags' into next

17eb11c CheckC*CompilerFlags: move common patterns to their own file
01cab89 CheckC*CompilerFlag: add documentation what to expect from a positive result
CheckC*CompilerFlag: add documentation what to expect from a positive result
Check*CompilerFlag: make C and CXX modules share most error patterns (#14339)

This simplifies maintenance as most patterns will be for both modules and may
be added at a central place.

While at it add error patterns for XL and BCC.
Merge topic 'cxx-flags' into next

10d4dc0 Check*CompilerFlag: make C and CXX modules share most error patterns (#14339)
293ecfe CheckC*CompilerFlag: add documentation what to expect from a positive result