FileCheck: fix a bug with multiple --check-prefix options. Similar to r194565

Summary:
Directives are being ignored, when they occur between a partial-word false
match and any match on another prefix.

For example, with FOO and BAR prefixes:
   _FOO
   FOO: foo
   BAR: bar
FileCheck incorrectly matches:
   fog
   bar

This happens because FOO falsely matched as a partial word at '_FOO' and was
ignored while BAR matched at 'BAR:'. The match of BAR is incorrectly returned
as the 'first match' causing the FOO directive to be discarded.

Fixed this the same way as r194565 (D2166) did for a similar test case.
The partial-word false match should be counted as a match for the purposes of
finding the first match of a prefix, but should be returned as a false match
using CheckTy::CheckNone so that it isn't treated as a directive.

Fixes PR17995

Reviewers: samsonov, arsenm

Reviewed By: samsonov

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2228