Split a chunk of -Wconditional-uninitialized warnings out into a separate flag,

-Wsometimes-uninitialized. This detects cases where an explicitly-written branch
inevitably leads to an uninitialized variable use (so either the branch is dead
code or there is an uninitialized use bug).

This chunk of warnings tentatively lives within -Wuninitialized, in order to
give it more visibility to existing Clang users.

llvm-svn: 157458