[llvm-reduce] Remove debug metadata elements

There can be lots of `MDTuple` debug metadata nodes. For example, `globals: !{!1, !2}` in `!DICompileUnit()`. Search through all debug info to find `MDTuple`'s and remove some of their elements.

For D135114 I was able to get a reproducer with 364 lines without manually deleting elements. After this patch I got it down to 67 lines.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D135237