DI: Cleanup comments, NFC
A number of comment cleanups:
- Remove duplicated function and class names from comments.
- Remove duplicated comments from source file (some of which were
out-of-sync).
- Move any unduplicated comments from source file to header.
- Remove some noisy comments entirely (e.g., a comment for
`DIDescriptor::print()` saying "print descriptor" just gets in the
way of reading the code).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219801
91177308-0d34-0410-b5e6-
96231b3b80d8