Support escaping in TrigramIndex.

Summary:
This is a follow up to r288303, where I have introduced TrigramIndex
to speed up SpecialCaseList for the cases when all rules are
simple wildcards, like *hello*wor.d*.

Here, I add support for escaping, so that it's possible to
specify rules like *c\+\+abi*.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27318

llvm-svn: 288553