GDScript: Fix constant conversions

[vonagam]

A constant value can be converted to a specified type during analysis, not at runtime every time.

var x: StringName = 'a' # initializer

x = 'b' # assignment

func print_name(y: StringName): print(y)
print_name('c') # call argument

func make_name() -> StringName: return 'd' # return

var z := 'c' as StringName # cast

Out of those places only variable initializer was handled at compile time before.

Initializer for constants was not converted at all which was leading to many crash reports.

While at it, allowed a cast to be considered as constant expression under certain circumstances (which will be expanded with array PR from which code for this PR partially comes from).

Fixed minor things with casts and arguments validations.

Fixes #70861.
Fixes #71255.
Fixes #71483.
Fixes #71635.
Fixes #72209.

Related to #70872 (supersedes).
Related to #70873 (supersedes).

[dalexeev]

This PR does not fully supersede #70873, as there are more changes to resolve_assignable(), not just the part that can be replaced by update_const_expression_builtin_type() call. (I will rebase #70873 after this PR is merged.)

[vonagam]

I looked now and assume that the thing that wasn't included here was a correction for wrong check of compatibility when it comes to asymmetric conversions - String can be converted to Color, but other way around will never work.

It can be fixed by disallowing implicit conversions in reverse check (reverse check is done only to allow cases where variable of class type is used where subclass is expected, since those are objects no implicit conversions should take place). Updated the PR and fixed the same mistake in usual assignments too.

Is there anything else that is missing?

[dalexeev]

Is there anything else that is missing?

1. Some tests are missing.
2. I think in my PR the code has been reorganized to be more readable: first the status is determined (SAFE, UNSAFE, ERROR) and then actions are performed depending on this. This can help avoid mistakes in the future.

[akien-mga]

Needs rebase.

[akien-mga]

Needs another rebase to pass CI, I fixed the GDScript test error.

[akien-mga]

Thanks!