3.6.1 Nomination: r232438, r232443, and clang/r232439, which fix pr22436

[Justin Bogner]

Hey Tom, can you cherry pick these three patches, which collectively fix
llvm.org/pr22436, to the 3.6 branch? I'm the code owner for profiling
and coverage instrumentation and this is a pretty straightforward bug
fix.

  llvm/r232438:

    GCOV: Make the exit block placement from r223193 optional
    
    By default we want our gcov emission to stay 4.2 compatible, which
    means we need to continue emit the exit block last by default. We add
    an option to emit it before the body for users that need it.

  llvm/r232443:

    llvm-cov: Warn instead of error if a .gcda has arcs from an exit block
    
    Patch by Vanderson M. Rosario. Thanks!

  clang/r232439:

    GCOV: Expose the -coverage-exit-block-before-body flag in clang -cc1
    
    This exposes the optional exit block placement logic from r232438 as a
    clang -cc1 option. There is a test on the llvm side, but there isn't
    really a way to inspect the gcov options from clang to test it here as
    well.