[clang-tidy] Improve string type matcher for abseil-string-find-starts-with check.

Summary:
This patch improves the check to match the desugared "string" type (so that it
can handle custom-implemented string classes), see the newly-added test.

Reviewers: alexfh

Subscribers: klimek, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D47704

llvm-svn: 334270