pgsql: Use appropriate -Wno-warning switches when compiling bitcode.

Use appropriate -Wno-warning switches when compiling bitcode.

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

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/dff01e40496122a9a4b3fb5a47ed5d124d32aff9

Modified Files
--------------
configure | 194 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
configure.in | 46 +++++++++++---
2 files changed, 222 insertions(+), 18 deletions(-)