Simplify find_first_of & find_last_of on single char.

Summary:
When calling find_first_of and find_last_of on a single character,
we can instead just call find / rfind and make our intent more
clear.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12518

llvm-svn: 246609