[CIR][CIRGen] Add CIRGen support for pointer-to-member-functions

[Lancern]

This PR adds the initial CIRGen support for pointer-to-member-functions. It contains the following new types, attributes, and operations:

• !cir.method, which represents the pointer-to-member-function type.
• #cir.method, which represents a literal pointer-to-member-function value that points to non-virtual member functions.
• #cir.virtual_method, which represents a literal pointer-to-member-function value that points to virtual member functions.
• cir.get_method_callee, which resolves a pointer-to-member-function to a function pointer as the callee.

See the new test at clang/test/CIR/CIRGen/pointer-to-member-func.cpp for how these new CIR stuff works to support pointer-to-member-functions.

[bcardosolopes]

Sorry for the delay here, taking a look next week!

[Lancern]

Rebased onto the latest main.

[Lancern]

Rebased onto the latest main.

[bcardosolopes]

Also one conflict