PR20625: Instantiate static constexpr member function of a local struct in a...

PR20625: Instantiate static constexpr member function of a local struct in a function template earlier.

This is necessary in order to allow the use of a constexpr member function, or
a member function with deduced return type, of a local class within a
surrounding instantiated function template specialization.

Patch by Michael Park!

This re-commits r236063, which was reverted in r236134, along with a fix for a
delayed template parsing bug that was exposed by this change.

llvm-svn: 237064