Merge topic 'warn-invalid-experimental'

9036aa10f3 Experimental: provide useful warning for invalid experimental variable values
b05e5403d9 Experimental: Avoid mutating statically allocated data
348ba701fe Tests/RunCMake/cmake_language: Expect experimental warnings more precisely

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !10767