PR50767: clear non-distinct debuginfo for function with nodebug definition...

PR50767: clear non-distinct debuginfo for function with nodebug definition after undecorated declaration

Fix suggested by Yuanfang Chen:

Non-distinct debuginfo is attached to the function due to the undecorated declaration. Later, when seeing the function definition and `nodebug` attribute, the non-distinct debuginfo should be cleared.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D104777