DebugInfo: Updates test cases that add method declarations.
We've investigated one reason why debugging information often goes wrong at https://reviews.llvm.org/D152095.
> LLVM can't handle IR where subprogram definitions are nested within DICompositeType when doing LTO builds, because there's no good way to cross the CU boundary to insert a nested DISubprogram definition in one CU into a type defined in another CU.
In #111167, we added a declaration for the DISubprogram for the method. This PR completes this test case.
stream history: https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm/topic/Dwarf.20CUs/near/384269475.
e6a5ff264b