[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.

llvm-svn: 292246