[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
diff --git a/llvm/lib/Analysis/TargetLibraryInfo.cpp b/llvm/lib/Analysis/TargetLibraryInfo.cpp
index 27e9803..aea29e6 100644
--- a/llvm/lib/Analysis/TargetLibraryInfo.cpp
+++ b/llvm/lib/Analysis/TargetLibraryInfo.cpp
@@ -1177,9 +1177,10 @@
             FTy.getParamType(1) == SizeTTy && FTy.getParamType(2) == SizeTTy);
 
   case LibFunc::NumLibFuncs:
-    llvm_unreachable("Invalid sentinel libfunc value");
+    break;
   }
 
+  llvm_unreachable("Invalid libfunc");
 }
 
 bool TargetLibraryInfoImpl::getLibFunc(const Function &FDecl,