MFC r265477: Merge -fstandalone-debug from Clang r198655:

  Implement a new -fstandalone-debug option. rdar://problem/15685848
  It controls everything that -flimit-debug-info used to, plus the
  vtable type optimization. The old -fno-limit-debug-info option is now an
  alias to -fstandalone-debug and vice versa.

  Standalone is the default on Darwin until dtrace is updated to work with
  non-standalone debug info (rdar://problem/15758808).

  Note: I kept the LimitedDebugInfo name in CodeGenOptions::DebugInfoKind
  because NoStandaloneDebugInfo sounded even more confusing.
MFC r266630 by dim:

  Add the clang patch for r265477.  While here, add a description to the
  patch for r263619, and unify all the URLs to point to svnweb
MFC r261799: Add bounds check for pteindex / pdeindex to amd64 kvm_minidump
MFC r265157: kldxref: Clean up error reporting

  Omit "too many sections" warnings if the ELF file is not dynamically
  linked (and is therefore skipped anyway), and otherwise output it only
  once.  An errant core file would previously cause kldxref to output a
  number of warnings.

  Also introduce a MAXSEGS #define and replace literal 2 with it, to make
  comparisons clear.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation