[modules] Properly look up the owning module for an instantiation of a merged template.

[modules] Properly look up the owning module for an instantiation of a merged template.

When looking for the template instantiation pattern of a templated entity,
consistently select the definition of the pattern if there is one. This means
we'll pick the same owning module when we start instantiating a template that
we'll later pick when determining which modules are visible during that
instantiation.

This reinstates r300650, reverted in r300659, with a fix for a regression
reported by Chandler after commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300938 91177308-0d34-0410-b5e6-96231b3b80d8