GCOV: Expose the -coverage-exit-block-before-body flag in clang -cc1

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@232439 91177308-0d34-0410-b5e6-96231b3b80d8