Add a log-invalid-remappings switch for ABI-illegal remappings

[tannergooding]

This adds a new opt-in --log-invalid-remappings config switch that emits a DiagnosticLevel.Info diagnostic for each configured remapping whose value is structurally not a legal unmanaged type name, scoped precisely to the array/fixed-buffer shape (a numeric subscript [<digit>, the same rule as the lowering, so a managed int[] with an empty subscript is left untouched).

The message names the remapping and states the applied lowering, e.g.:

Potential invalid remapping 'MyBuffer=sbyte[8]'. A fixed-size-buffer type is not directly expressible in an unmanaged signature; it was lowered to the pointer 'sbyte*'.

The lowering used to compute the shown pointer is factored into a shared TryLowerFixedSizeBufferRemapping helper so the diagnostic and GetRemappedTypeName can't drift.

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This builds on #745 (the fixed-size-buffer-to-pointer lowering), which has since merged; the branch is based directly on that commit so the diff here is just the diagnostic feature.

[tannergooding]

Dropping this — not high enough value relative to the number of ABI edge cases it would need to cover to be trustworthy.