gccrs: fix parser error on parenthesised types

[Villosse]

Do not cast parenthesised types to TraitBound types

Fixes #4148

gcc/rust/ChangeLog:

* parse/rust-parse-impl.h (Parser::parse_paren_prefixed_type):

gcc/testsuite/ChangeLog:

* rust/compile/issue-4148.rs:

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