[llvm] Do not replace dead constant references in metadata with undef

This patch removes an incorrect behaviour in Constants.cpp, which would
replace dead constant references in metadata with an undef value. This
blanket replacement resulted in undef values being inserted into
metadata that would not accept them. The replacement was intended for
debug info metadata, but this is now instead handled in the RAUW
handler.

Differential Revision: https://reviews.llvm.org/D117300