FileCheck: fix a bug with multiple --check-prefix options.

FileCheck: fix a bug with multiple --check-prefix options.

Summary:
This fixes a subtle bug in new FileCheck feature added
in r194343. When we search for the first satisfying check-prefix,
we should actually return the first encounter of some check-prefix as a
substring, even if it's not a part of valid check-line. Otherwise
"FileCheck --check-prefix=FOO --check-prefix=BAR" with check file:

  FOO not a vaild check-line
  FOO: foo
  BAR: bar

incorrectly accepted file:

  fog
  bar

as it skipped the first two encounters of FOO, matching only BAR: line.

Reviewers: arsenm, dsanders

Reviewed By: dsanders

CC: llvm-commits
Differential Revision: http://llvm-reviews.chandlerc.com/D2166

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194565 91177308-0d34-0410-b5e6-96231b3b80d8