Summary:
This allows them to be used without -cc1 the same way as -I and -isystem.
Renamed the options to --system-header-prefix=/--no-system-header-prefix to avoid interference with -isystem and make the intent of the option cleaner.

Reviewers: rsmith

Reviewed By: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3185

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204775 91177308-0d34-0410-b5e6-96231b3b80d8