[llvm][DebugInfo] Attach object-pointer to DISubprogram declarations (#122742)

Currently Clang only attaches `DW_AT_object_pointer` to
`DW_TAG_subprogram` definitions. LLDB constructs C++ method types from
their `DW_TAG_subprogram` declaration, which is also the point at which
it needs to determine whether a method is static or not. LLDB's
heuristic for this could be very simple if we emitted
`DW_AT_object_pointer` on declarations. But since we don't, LLDB has to
guess whether an argument is an implicit object parameter based on the
DW_AT_name and DW_AT_type.

To simplify LLDB's job (and to eventually support C++23's explicit
object parameters), this patch adds the `DIFlagObjectPointer` to
`DISubprogram` declarations.

For reference, GCC attaches the object-pointer DIE to both the
definition and declaration: https://godbolt.org/z/3TWjTfWon

Fixes https://github.com/llvm/llvm-project/issues/120973

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