in GCC and LLVM build configs

This was accomplished by adding GccStyleFilterAndCombine() in copts.py

Previously, if we had a default warning of the form -Wwarning, adding
-Wno-warning to the list of test warnings would just add conflicting
flags. We now filter -Wwarning if -Wno-warning is added to the test warnings.

PiperOrigin-RevId: 495683815
Change-Id: I5dfd8a30b0be09d6b48237f61d598230ab9027db