Avoid SourceManager.h include in RawCommentList.h, add missing incs

SourceManager.h includes FileManager.h, which is expensive due to
dependencies on LLVM FS headers.

Remove dead BeforeThanCompare specialization.

Sink ASTContext::addComment to cpp file.

This reduces the time to compile a file that does nothing but include
ASTContext.h from ~3.4s to ~2.8s for me.

Saves these includes:
    219 -    ../clang/include/clang/Basic/SourceManager.h
    204 -    ../clang/include/clang/Basic/FileSystemOptions.h
    204 -    ../clang/include/clang/Basic/FileManager.h
    165 -    ../llvm/include/llvm/Support/VirtualFileSystem.h
    164 -    ../llvm/include/llvm/Support/SourceMgr.h
    164 -    ../llvm/include/llvm/Support/SMLoc.h
    161 -    ../llvm/include/llvm/Support/Path.h
    141 -    ../llvm/include/llvm/ADT/BitVector.h
    128 -    ../llvm/include/llvm/Support/MemoryBuffer.h
    124 -    ../llvm/include/llvm/Support/FileSystem.h
    124 -    ../llvm/include/llvm/Support/Chrono.h
    124 -    .../MSVCSTL/include/stack
    122 -    ../llvm/include/llvm-c/Types.h
    122 -    ../llvm/include/llvm/Support/NativeFormatting.h
    122 -    ../llvm/include/llvm/Support/FormatProviders.h
    122 -    ../llvm/include/llvm/Support/CBindingWrapping.h
    122 -    .../MSVCSTL/include/xtimec.h
    122 -    .../MSVCSTL/include/ratio
    122 -    .../MSVCSTL/include/chrono
    121 -    ../llvm/include/llvm/Support/FormatVariadicDetails.h
    118 -    ../llvm/include/llvm/Support/MD5.h
    109 -    .../MSVCSTL/include/deque
    105 -    ../llvm/include/llvm/Support/Host.h
    105 -    ../llvm/include/llvm/Support/Endian.h

Reviewed By: aaron.ballman, hans

Differential Revision: https://reviews.llvm.org/D75279