[Chapel Merge] For clang, move `-W[missing|strict]-prototypes` to `WARN_CXXFLAGS` from `WARN_CFLAGS`

Branch: refs/heads/main
Revision: deafadeb00a3f5758355c93990b7149cd5e14767
Author: bradcray
Link: For clang, move `-W[missing|strict]-prototypes` to `WARN_CXXFLAGS` from `WARN_CFLAGS` by bradcray · Pull Request #21940 · chapel-lang/chapel · GitHub
Log Message:
For clang, move -W[missing|strict]-prototypes to WARN_CXXFLAGS from WARN_CFLAGS (#21940)

...so that they will apply to both C++ and C compiles.

[reviewed by @riftEmber]

The motivation for this is that our CI testing was catching things with
g++ that we weren't catching as developers using clang++. This should
bring the clang++ warnings more in-line with what we're getting from
g++.

It looks to me as though it's been this way since we first added support
for clang, so putting them in both was either an oversight, or perhaps
they weren't supported by clang++ back then. I also get the sense
(though I didn't look into it very carefully) that g++ and clang++ may
have slightly different warnings associated with different flags in
their C/C++ compilers. In any case, my guess is that this will help any
developers using warnings in recent versions of clang++, but please
alert me if that's incorrect.

Compare: Comparing c69f8015084aa5c9c6af401a86a601c7e741749c...267fd3672eca3ab92736adf8d2fc35ddd21c7aec · chapel-lang/chapel · GitHub

Diff:
M make/compiler/Makefile.clang
https://github.com/chapel-lang/chapel/pull/21940.diff