Emit const globals with constexpr destructor as constant LLVM values

This follows 2b4fa534 which made Clang not emit destructor calls for such
objects. However, they would still not get emitted as constants since
CodeGenModule::isTypeConstant() returns false if the destructor is
constexpr. This change adds a param to make isTypeConstant() ignore the
dtor, allowing the caller to check it instead.

Fixes Issue #61212

Differential revision: https://reviews.llvm.org/D145369