Recommit r179497 after fixing uninitialized variable.
Revert "Recommit r179497 after fixing uninitialized variable." until
I can fix the testcases here:

http://lab.llvm.org:8011/builders/clang-native-arm-cortex-a9/builds/6952

This reverts commit r179512 due to testcases specifying triples
that they didn't actually mean and causing failures on other platforms.
Add triples to these tests since they're now using TLS, which isn't available on all targets.
Add triple to another test.
Properly check for a constant initializer for a thread-local variable.
Local thread_local variables are implicitly 'static'. (This doesn't apply to _Thread_local nor __thread.)