gccrs: port over readonly_error from c-family for lvalue assignment checks
Fixes #2391
gcc/rust/ChangeLog:
* Make-lang.in: fixup formatting
* resolve/rust-ast-resolve-expr.cc (ResolveExpr::visit): remove old check
* rust-session-manager.cc (Session::compile_crate): call new lint
* resolve/rust-ast-verify-assignee.h: Removed.
* checks/errors/rust-readonly-check.cc: New file.
* checks/errors/rust-readonly-check.h: New file.
gcc/testsuite/ChangeLog:
* rust/compile/wrong_lhs_assignment.rs: update error message
* rust/compile/issue-2391.rs: New test.
Signed-off-by: Philip Herron <herron.philip@googlemail.com>