Fix compilation warnings and re-enable werror=yes on Travis

Fix -Wunused-variable, -Wunused-but-set-variable and -Wswitch warnings
raised by GCC 8 and 9.

Fix -Wunused-function, -Wunused-private-field and
-Wtautological-constant-out-of-range-compare raised by Clang.

Fix MSVC 2019 warning C4804 (unsafe use of type 'bool' in comparison
operation).

GCC -Wcpp warnings/Clang -W#warnings (`#warning`) are no longer raising
errors and will thus not abort compilation with `werror=yes`.

Treat glslang headers are system headers to avoid raising warnings.

Re-enables us to build with `werror=yes` on Linux and macOS, thus
catching warnings that would be introduced by new code.

Fixes #36132.

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