docs: Update LangRef and SourceLevelDebugging

Cleanup some bitrot in SourceLevelDebugging.rst.

  - Pull the still-relevant details about individual descriptors into
    LangRef.rst.  Cut a lot of it to avoid over-describing the fields,
    as the C++ classes and assembly format are mostly self-describing
    now.  If there's anything specific that I shouldn't have cut, let me
    know and I'll add it back.
  - Rewrite the remaining sections to refer to the new debug info
    hierarchy in LangRef.rst.

llvm-svn: 232566