[clang-tidy] Fix treating non-space whitespaces in checks list.

Summary:
This furtherly improves r295303: [clang-tidy] Ignore spaces between globs in the Checks option.
Trims all whitespaces and not only spaces and correctly computes the offset of the checks list (taking the size before trimming).

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: cfe-commits, JDevlieghere

Differential Revision: https://reviews.llvm.org/D30567

llvm-svn: 298621