checker: fix generic fn with nested generic fn call (fix #18285)

[yuyi98]

This PR fix generic fn with nested generic fn call (fix #18285).

• Fix generic fn with nested generic fn call.
• Add test.

struct Test {}

fn unmarshal[T]() ! {
	get_number[int]()!
}

fn get_number[T]() !T {
	return T(42)
}

fn main() {
	unmarshal[Test]()!
	assert true
}

PS D:\Test\v\tt1> v run .

[prantlf]

I can confirm that the fix works well.

A function call with an explicit generic type:

fn unmarshal_number[T](a Any) !T { ... }

fn unmarshal_struct[T](mut dst T, anysrc Any) ! {
  src := anysrc.object()!
  $for field in T.fields {
    anyval := src[field.name]!
    $if field.typ is int {
      dst.$(field.name) = unmarshal_number[int](anyval)!
    }
    ...

A function call with an inferred generic type:

fn convert_number[T](float f64, num T) !T { ... }

fn unmarshal_struct[T](mut dst T, anysrc Any) ! {
  src := anysrc.object()!
  $for field in T.fields {
    anyval := src[field.name]!
    $if field.typ is int {
      num := anyval.number()!
      dst.$(field.name) = convert_number(num, int(num))!
    }
    ...

I'm looking forward to seeing this fix merged :-)

[prantlf]

Damn' fast you were :-) Thank you!