internal: less generic-expression workarounds

[zerbina]

Summary

Prevent expressions-depending-on-unresolved-type-variables passed to
tryConstExpr from reaching into the compile-time evaluation machinery.
This allows for removing multiple workarounds from mirgen and vmgen.

Details

Before tryConstExpr passes the expression to evalConstExpr, it now
first checks whether the expression either contains unresolved generic
parameters or is an unresolved type-parameter-lookup (e.g., T.param).
If either is the case, nil (which signals that the expression cannot
be evaluated at compile-time) is returned without passing the
expression to evalConstExpr.

While this approach is fundamentally also a workaround (expressions
containing unresolved generic parameters shouldn't be passed to
tryConstExpr in the first place), it at least prevents unresolved
expressions from reaching into transf, mirgen, and vmgen.

mirgen now doesn't have to push unresolved generic parameters or
type-parameter-lookups as mnkLiterals through the MIR phase, and
vmgen doesn't have to special-case them. More generally, vmgen can
now treat unexpected nodes reaching it as a defect (via unreachable).

Since vmGenDiagCodeGenUnexpectedSym and vmGenDiagCannotGenerateCode
are now unused, they are, together with everything related to them,
removed.

[saem]

Wow, it's surprising how much that code proliferated.

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• yet another preparation for compiler: introduce an IR for the code generators #551