[gtest] Detect warning flags using the positive spelling.

[gtest] Detect warning flags using the positive spelling.

Some GCC versions will accept any warning flag name after a '-Wno-',
which would cause us to try to disable warnings with names GCC didn't
understand. This will silently succeed unless there is some other output
from GCC in which case we get weird cc1plus warnings about the warning
name being bogus.

There is still the issue that gtest sets warning flags for building
gtest-all.cc using weird 'add_definitions' and the fact that there is
a GCC version which warns on the variadic macro usage in gtest under
-pedantic, but has no flag analogous to Clang's
-Wgnu-zero-variadic-macro-argumnets to suppress this warning. I haven't
been able to come up with any good solution here. The closest is to turn
off -pedantic for those versions of GCC, but that seems really nasty.
For now, those versinos of GCC aren't warning clean. If anyone is broken
by this, I'll work on CMake logic to detect and disable -pedantic in
these cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291299 91177308-0d34-0410-b5e6-96231b3b80d8