Fix two MSVC warnings; NFC

One was "unsafe use of bool" and the other was "sign comparision
mismatch", and both were because we're treating a bool object as if it
were an unsigned int. Add a cast to make that more explicit.

Author: AaronBallman

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -5729,7 +5729,7 @@ static bool isAtLeastAsSpecializedAs(
                       TemplateDeductionInfo &Info,
                       SmallVectorImpl<DeducedTemplateArgument> &Deduced,
                       PartialOrderingKind) {
-                    if (ArgIdx && *ArgIdx >= Args1Offset)
+                    if (ArgIdx && *ArgIdx >= static_cast<unsigned>(Args1Offset))
                       ArgIdx = *ArgIdx - Args1Offset;
                     else
                       ArgIdx = std::nullopt;