don't typecheck untyped + allow void typed template param default values

[metagn]

Previously, the compiler never differentiated between untyped/typed argument default values and other types, it considered any parameter with a type as typed and called semExprWithType, which both typechecked it and disallowed void expressions. Now, we perform no typechecking at all on untyped template param default values, and call semExpr instead for typed params, which allows expressions with void type.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 4eed341

Hint: mm: orc; opt: speed; options: -d:release
174662 lines; 8.111s; 653.801MiB peakmem