fix Issue 15550 - compile error while testing template constraint

[MartinNowak]

• fixing typing of incompletely instantiated function templates
  (don't use void)
• use TypeInstance so it will fail w/ a proper error message

[dlang-bot]

Fix	Bugzilla	Description
✓	15550	[Reg 2.070.0-b1] compile error while testing template constraint

[MartinNowak]

This is a bit more difficile. We must not generate an error during ScopeExp.semantic, b/c the partially specialized template can be used later on. On the other hand we have to generate an instantiation error when the type of such a ScopeExp is used.

[MartinNowak]

It's too risky to fix this at the end of the beta. It's also a deeper problem, because ScopeExp can't know whether it's used in a function call with arguments or in a typeof expression, it's not possible to resolve the template instance only within ScopeExp. Subsequently ScopeExp should be what it's doc comment says Mainly just a placeholder, and do nothing (or better assert(0)) in it's semantic function.
Anything involving a ScopeExp must be handled above it (using the template instance stored in the ScopeExp).