Fix ICE by rejecting const blocks in patterns during AST lowering (closes#148138)
This PR fixes the ICE reported in rust-lang/rust#148138.
The root cause is that `const` blocks aren’t allowed in pattern position, but the AST lowering logic still attempted to create `PatExprKind::ConstBlock`, allowing invalid HIR to reach type checking and trigger a `span_bug!`.
Following the discussion in the issue, this patch removes the `ConstBlock` lowering path from `lower_expr_within_pat`. Any `ExprKind::ConstBlock` inside a pattern is now handled consistently with other invalid pattern expressions.
A new UI test is included to ensure the compiler reports a proper error and to prevent regressions.
Closesrust-lang/rust#148138.
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