checker: fix generic fn infering map argument (fix #18337)

[yuyi98]

This PR fix generic fn infering map argument (fix #18337).

• Fix generic fn infering map argument.
• Add test.

fn f[T](src map[string]T) T {
	return src['a']
}

fn main() {
	r1 := f({
		'a': 1
	})
	println(r1)
	assert r1 == 1

	r2 := f({
		'a': 'hello'
	})
	println(r2)
	assert r2 == 'hello'
}

PS D:\Test\v\tt1> v run .
1
hello

[prantlf]

I can confirm that this fix works. Thank you!

The real code is a factory function taking any map as input:

r := any_object({
  'a': 1
})
``