Rollup merge of #111167 - cuviper:type-decl-disubprogram, r=michaelwoerister

debuginfo: split method declaration and definition

When we're adding a method to a type DIE, we only want a DW_AT_declaration
there, because LLVM LTO can't unify type definitions when a child DIE is a
full subprogram definition. Now the subprogram definition gets added at the
CU level with a specification link back to the abstract declaration.

Both GCC and Clang write debuginfo this way for C++ class methods.

Fixes #109730.
Fixes #109934.

Author: matthiaskrgr