Revert commit 5b45a3d0ce (CheckCompilerFlag: Match the Clang "argument
unused" output for all languages, 2023-01-23, v3.26.0-rc1~38^2). It
broke existing projects that were silently tolerating unrelated unused
arguments in their checks for C and CXX. For example, using
`CFLAGS=-nostdinc` or `CXXFLAGS=-nostdinc++` causes those flags to be
used when driving the linker as well, and Clang warns they are unused in
that case.
Add a test case covering the now-restored behavior.
Fixes: #24591
Improve detection of missing compiler flags: move "argument unused
during compilation: .*" pattern from language-specific branches into
the common list.
Fix a typo from commit 660e0d80ae (internal/CheckCompilerFlag: rely on
common configuration, 2022-01-12, v3.23.0-rc1~124^2~1) that caused
locale environment variables to not be restored after they are set
during the check.
