When two templates get merged together, also merge their pattern declarations

When two templates get merged together, also merge their pattern declarations
together. This is extremely hairy, because in general we need to have loaded
both the template and the pattern before we can determine whether either should
be merged, so we temporarily violate the rule that all merging happens before
reading a decl ends, but *only* in the case where a template's pattern is being
loaded while loading the template itself.

In order to accomodate this for class templates, delay loading the injected
class name type for the pattern of the template until after we've loaded the
template itself, if we happen to load the template first.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207063 91177308-0d34-0410-b5e6-96231b3b80d8