Ensure __config_site definitions are passed to modules tests.
Ensure __config_site definitions are passed to modules tests.

The test configuration contained a bug where we only raised
the __config_site commands to the command line if modules were
enabled for all of the libc++ tests. However there are special
modules-only tests, and these tests weren't getting the correct
defines.

This patch corrects that issue.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@367267 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx