Merge topic 'modules-better-messages'

571b5e1f2c cxxmodules: improve error messages for C++ module setup
8b4d32c18b cmStandardLevelResolver: add query for the effective standard level
17ddc4ac76 cmStandardLevelResolver: compare with static string literals
6f1dae2b01 cmStandardLevelResolver: use `cmStrCat` where possible
0d45d40e13 cmStandardLevelResolver: use character literals where possible

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !8755