[clang] [CIR] Add invariant attribute to cir.load (PR #204283)

[via cfe-commits]

skc7 wrote:

> Is it possible to add a test that generates this attribute from C/C++ code?

`CIRGenItaniumCXXABI::getVirtualFunctionPointer` now marks the virtual function-pointer load invariant under `-O1 -fstrict-vtable-pointers`, mirroring classic CodeGen (ItaniumCXXABI.cpp).
Added clang/test/CIR/CodeGen/vtable-load-invariant.cpp test for this.

https://github.com/llvm/llvm-project/pull/204283