Implement -Wvla correctly

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