gccrs: add path statement lint

[lucasly-ba]

This patch adds the path statement lint, which warns on a statement made of a single path expression, as it has no effect.

gcc/testsuite/ChangeLog:

* rust/compile/path-statements_0.rs: New test.

[powerboat9]

Does this catch cases where a path statement is used, and has an effect? Say,

mod a {
    pub(crate) const B: bool = true;
}

pub fn foo_is_active () -> bool {
    a::B
}

[lucasly-ba]

Does this catch cases where a path statement is used, and has an effect? Say,

mod a {
    pub(crate) const B: bool = true;
}

pub fn foo_is_active () -> bool {
    a::B
}

Yes. The lint only fires on a path in statement position, a path used as a tail/return expression like your a::B isn't an ExprStmt, so it's never flagged same as rustc's path_statements. I've added your example as a test to make that explicit.