Provide fixit for inferred property of opaque type (#69241)

[Androp0v]

When a pattern binding at top level attempts to pick up another declaration's opaque result type as its type by type inference, provide a fixit to explicitly declare the type. This is done to prevent opaque result types from propagating nontrivially into other declarations' types, see 6db0540

(First PR in this repo 🙏)

Added fix-it that annotates a top level declaration property declaration of inferred opaque type to be explicitly typed. Reasoning for this requirement in 6db0540.

Resolves #69241

Tests

Reused the 3 existing cases covered in test/type/opaque.swift:

Test case 1

let inferredOpaque = bar()

Is fixed to:

let inferredOpaque: some P = bar()

Test case 2

let inferredOpaqueStructural = Optional(bar())

Is fixed to:

let inferredOpaqueStructural: Optional<some P> = Optional(bar())

Test case 3

let inferredOpaqueStructural2 = (bar(), bas())

Is fixed to

let inferredOpaqueStructural2: (some P, some P & Q) = (bar(), bas())