The regex for -isystem matching is broken. -[D,I,Usystem] matches "-D", "-,",
"-I", "-U", "-s" "-y", etc. Besides that, "-isystem /foo" gets interpreted as
"-i" with a non-empty value "system" and thus the next "/foo" argument is not
read. This patch corrects the regex.

This fixes PR13237 <https://llvm.org/bugs/show_bug.cgi?id=13237>.

A patch by Peter Wu!

Differential Revision: http://reviews.llvm.org/D13800

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@251312 91177308-0d34-0410-b5e6-96231b3b80d8