The latter already have similar checks for similar modules.

Cases for `CheckSource{Compiles,Runs}` and `Check{Compiler,Linker}Flag`
cover the modules with the language as a parameter, not the
language-specific modules.

Fix the language checks added by commit 90dead024c (CheckCompilerFlag:
unified way to check compiler flags per language, 2020-09-25,
v3.19.0-rc1~88^2), commit 10ae907de0 (CheckSoureCompiles: Add a unified
way to check if a source compiles, 2020-09-14, v3.19.0-rc1~118^2~1), and
commit 357e2ef429 (CheckSoureRuns: Add a unified way to check if a
source runs, 2020-09-14, v3.19.0-rc1~118^2) to work when variables of
the language names are set.

Fixes: #21500

The old `check_X_source_runs` modules did not verify the arguments, so
we cannot start doing it now. Downgrade the hard error introduced in
commit 357e2ef429 (CheckSoureRuns: Add a unified way to check if a
source runs, 2020-09-14) via !5223 into a noisy warning.

The conversion of Check<Lang>CompilerFlag, SourceCompiles, and
SourceRuns over to the new functions has the possibility of breaking
projects that had functions with those existing names.

To reduce the possibility of collisions we now have all the
legacy code call functions that start with `cmake_`, and users
will need to explicitly include the new modules to get the
non-prefixed versions

Fixes: #21359