[Utils] Salvage debug info from all no-op casts

We already try to salvage debug values from no-op bitcasts and inttoptr
instructions: we should handle ptrtoint instructions as well.

This saves an additional 24,444 debug values in a stage2 build of clang,
and (according to llvm-dwarfdump --statistics) provides an additional
289 unique source variables.

llvm-svn: 324982