[include-fixer] make HeaderMapCollector maps from regex instead of postfix.

Summary:
this enables us to map a group of headers to one header name,
e.g. headers from one directory can be mapped to the same header.

Reviewers: bkramer

Subscribers: cfe-commits, hokein

Differential Revision: http://reviews.llvm.org/D21787

llvm-svn: 274494