Add support for building with _GLIBCXX_DEBUG. New configure option

Add support for building with _GLIBCXX_DEBUG.  New configure option
--enable-expensive-checks allows the developer to enable runtime
checking that can greatly increase compile time.  Currently it only
turns on _GLIBCXX_DEBUG.  Other expensive debugging checks added later
should be controlled by this configure option.

This patch also updates llvm-config with a --cppflags option to inform
llvm-gcc how to build itself so that it is compatible with an llvm that
was built with _GLIBCXX_DEBUG.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37777 91177308-0d34-0410-b5e6-96231b3b80d8