[libc++] Prevent stale site configuration headers

If we define cmake macros that require a site config, and then undefine
all such macros, a stale site config header will be left behind.
Explicitly delete any generate site config if we don't need one to avoid
this.

Differential Revision: https://reviews.llvm.org/D36720

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@313284 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed