[mlir][llvm] A DICompositeTypeAttr is also a DIScopeAttr.

A DICompositeTypeAttr is the MLIR counterpart of LLVMIR's
DICompositeType debug metadata node. It can be used to store debug
information associated to a class or struct type definition. Any class
member method then sets its scope to the parent DICompositeType. This
revision adds DICompositeTypeAttr to the list of DIScopeAttrs to
prevent verification failures for legal debug information. Additionally,
it adds a number of test cases.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D141480