Use -Wno-vla-extension globally.

Summary:
This was previously only established within debugserver, but
there is a use of the VLA extension in source/Host/macosx/Symbols.cpp,
so ignore this warning globally.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12515

llvm-svn: 246605