[DebugInfo] Move constructor homing case in shouldOmitDefinition.

For some reason the ctor homing case was before the template
specialization case, and could have returned false too early.
I moved the code out into a separate function to avoid this.

Also added a run line to the template specialization test. I guess
all the -debug-info-kind=limited tests should still pass with =constructor,
but it's probably unnecessary to test for all of those.

Differential Revision: https://reviews.llvm.org/D86491