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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157458 91177308-0d34-0410-b5e6-96231b3b80d8