Handle array type patterns in switch statements

[gafter]

In the proposed changes for C# 9.0, a type expression such as A[] can in theory be used as a switch case.

    case A[]:

However, it is likely that this will parse as an expression. The binder should be flexible enough to take the parse tree (for the expression) and bind it as a type, and the pattern binder should be capable of backing off from the expression interpretation to the type interpretation.

Similarly

    case (A[]):

This should have the same meaning as above, but currently parses as an erroneous cast expression.

[gafter]

Fixed in #40503