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).

llvm-svn: 219801