Improve code generation for thread_local variables:

Summary:
 * Don't bother using a thread wrapper when the variable is known to
   have constant initialization.
 * Emit the thread wrapper as discardable-if-unused in TUs that don't
   contain a definition of the thread_local variable.
 * Don't emit the thread wrapper at all if the thread_local variable
   is unused and discardable; it will be emitted by all TUs that need
   it.

Reviewers: rjmccall, jdoerfert

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67429

llvm-svn: 371767