[clang-tidy] Fix definitions in headers check to respect qualifiers

Summary:
The check was generating a fix without taking qualifiers in return type
into account. This patch changes the insertion location to be before qualifers.

Reviewers: gribozavr

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67213

llvm-svn: 371022