Parse: Disable delayed template parsing for constexpr functions

Commit r191484 treated constexpr function templates as normal function
templates with respect to delaying their parsing.  However, this is
unnecessarily restrictive because there is no compatibility concern with
constexpr, MSVC doesn't support it.

Instead, simply disable delayed template parsing for constexpr function
templates.  This largely reverts the changes made in r191484 but keeps
it's unit test.

This fixes PR17661.

llvm-svn: 193274