Don't recover &raw EXPR as a missing comma

[qaijuang]

The parser already has a targeted suggestion for &raw EXPR, added in #139392, telling the user that &raw must be followed by const or mut.

In comma-separated expression lists, the generic missing-comma recovery would then treat the expression after raw as the next list element. That produced follow-up diagnostics like:

• help: missing ','
• cannot find value raw in this scope
• an arity error for calls such as takes_raw_ptr(&raw x)

This PR avoids that comma recovery when we are already in the &raw missing-const/mut diagnostic path. For function calls, it preserves the call expression with a single error argument, so useful later diagnostics such as cannot find function f are still emitted.

Fixes #157015.

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @jieyouxu

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[jieyouxu]

(Focusing on debuginfo/compiletest/rustfmt reviews this weekend)
@rustbot reroll