Currently we would unconditionally add an implicit `this` parameter when
creating an instance method type. However, when we have an explicit
'this', we shouldn't generate one. This patch only passes a valid
`ThisPtr` type to `getOrCreateInstanceMethodType` if one wasn't
explicitly specified. There's no way to get a pointer to a member
function with an explicit `this` parameter (those are treated as regular
function pointers instead). So there's no need to account for that case
in `CGDebugInfo::CreateType`.
Fixes https://github.com/llvm/llvm-project/issues/99744
