[analyzer] Use the signature of the primary template for issue hash calculation

Now when a template is instantiated more times and there is a bug found in the
instantiations the issue hash will be different for each instantiation even if
every other property of the bug (path, message, location) is the same.

This patch aims to resolve this issue. Note that explicit specializations still
generate different hashes but that is intended.

Differential Revision: https://reviews.llvm.org/D38728

llvm-svn: 316900