[TLI] Appease spurious MSVC warning using llvm_unreachable. NFC.

r292188 confused MSVC because of the combined lack of a default
case and return statement.

Move the unreachable outside of the NumLibFuncs case, to make it
obvious that all cases should be handled.

llvm_unreachable is __declspec(noreturn), so I'm assuming this
does appease MSVC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292246 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed