Add locales to available_features for tests.

Linux has a lot of failures caused by not having support for certain
locales. Since these come out as a lot of noise in the test results,
have lit.cfg detect the presence of the various locales used in the
tests and add them to config.available_features as locale.LOCALE_NAME.

This patch also adds REQUIRES: locale.REQUIRED_LOCALE to every test that
I saw failing in this manner. We probably need to add more for all the
tests requiring en_US.UTF-8, but we can do that on an as-needed basis.

One thing that concerns me is how many tests get skipped because of
missing locales (especially in regex/). We should make a point of
splitting up any tests that test default behavior _and_ behavior under a
given locale so that we aren't losing coverage for default behavior.

llvm-svn: 214753