C++ feature checks: Match warnings more strictly

Require the word "warning" to appear at the start of a line, after
whitespace, or after a `:`.  This is the same that CTest launchers use
to match warnings.  It avoids matching "warning" inside file paths.

Fixes: #19019
6 jobs for <span class="ref-name">cxx-checks-warning-match</span>
Status Job ID Name Coverage
  External
passed ghostflow-check-master

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