Convert an llvm_unreachable in an 'else' block to a removal of the 'if' and an assertion of its condition. Suggestion from David Blaikie.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211142 91177308-0d34-0410-b5e6-96231b3b80d8
Replace some assert(0)'s with llvm_unreachable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211143 91177308-0d34-0410-b5e6-96231b3b80d8