[PATCH] -fms-extensions: Implement half of #pragma init_seg

[David Majnemer]

Could you add a `declspec(thread)`  testcase?

================
Comment at: lib/CodeGen/CGDeclCXX.cpp:276
@@ +275,3 @@
+
+  // LinkOnce and Weak linkage are lowered down a single-member comdat group.
+  // Make an explicit group so we can join it.
----------------
... lowered down **to** a single-member ...

http://reviews.llvm.org/D4549