Merge commit 21e5e1724b75 from llvm git:

Merge commit 21e5e1724b75 from llvm git:

  getMainExecutable: Fix hand-rolled AT_EXECPATH for older FreeBSD

  Once we hit AT_NULL, we need to bail out of the loop; not just the
  enclosing switch.  This fixes basic usage (e.g. `cc --version`) when
  AT_EXECPATH isn't present on older branches (e.g. under
  emu-user-static, at the moment), where we would previously run off
  the end of ::environ.

  Patch By: kevans

  Reviewed By: arichardson

  Differential Revision:  https://reviews.llvm.org/D79239

MFC after:	3 days