Nullness issue - add contextual information into nullness mismatches

[T-Gro]

Issue description

As of now, nullness related errors are coming from comparing pairs of types in the constraint solver, and all the nullness warnings come from here.

Which means that context (does it come from generic code, equality check, pattern matching, dereferencing a property via dot-lookup,... and many others) is not kept until the nullness constraint solving.

Which in the end makes the messages not user friendly - the ranges fit, but the messaging does not reflect context and source of the nullness mismatch.

Example:

let getLength (x: string | null) = x.Length

Choose one or more from the following categories of impact

• Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
• Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
• Breaking change related to older null constructs in code not using the checknulls switch.
• Breaking change related to generic code and explicit type constraints (null, not null).
• Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
• C#/F# interop issue related to nullness metadata.
• Other (none of the categories above apply).

Operating System

Windows (Default)

What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

.NET Runtime/SDK version

No response

Reproducible code snippet and actual behavior

No response

Possible workarounds

No response

[T-Gro]

(Hint: For method calls, this would be coming from CanMemberSigsMatchUpToCheck in the subsumeTypes invocation.
From there the info needs to go to ConstraintSolver to be known at warning time.