We use "clang" to compile bitcode files for LLVM inlining.  That might
be different from the build's main C compiler, so it needs its own set
of compiler flags.  To simplify configure, we don't bother adding any
-W switches to that flag set; there's little need since the main build
will show us any warnings.  However, if we don't want to see unwanted
warnings, we still have to add any -Wno-warning switches we'd normally
use with clang.

This escaped notice before commit 9ff47ea41, which tried to add
-Wno-compound-token-split-by-macro; buildfarm animals using mismatched
CC and CLANG still showed those warnings.  I'm not sure why we never
saw any effects from the lack of -Wno-unused-command-line-argument
(maybe that's only activated by -Wall?).  clang does not currently
support -Wno-format-truncation or -Wno-stringop-truncation, although
in the interests of future-proofing and consistency I included tests
for those.

Back-patch to v11 where we started building bitcode files.

Discussion: https://postgr.es/m/2921539.1637254619@sss.pgh.pa.us