GCC implements -Wvla as "warn on every VLA" (this is useful to find every VLA,
for example, if they are forbidden by coding guidelines). Currently Clang
implements -Wvla as "warn on VLA when it is an extension".
The attached patch makes our behavior match GCC. The existing vla extwarn is
moved under -Wvla-extension and is still included into -Wgnu.
This fixes PR5953.
llvm-svn: 173286
Dmitri Gribenko