gccrs: resolve: Add "break rust" Easter egg

gccrs: resolve: Add "break rust" Easter egg

When we encounter a "break rust" statement, emit a funny error message
and intentionally cause an ICE. This matches the corresponding Easter
egg in rustc. As a GNU extension, "break gcc" is also supported.

The conditions for this to happen are:
* The break expression must be literally "rust" or "gcc". For instance,
  "break (rust)" will not trigger the Easter egg.
* The name ("rust" or "gcc") must not be in scope; if it is, no error
  is emitted, and the compilation proceeds as usual. In other words,
  this only affects how GCC diagnoses programs that would fail to
  compile anyway.

Note that this is different from the conditions under which rustc emits
its ICE. For rustc, it matters whether or not the "break" is inside a
loop, and for us it matters whether or not the name resolves. The end
result should be the same anyway: valid programs continue to compile,
and typing in

fn main() {
    break rust;
}

triggers a funny ICE.

Closes https://github.com/Rust-GCC/gccrs/issues/1996

gcc/rust/ChangeLog:
	* resolve/rust-ast-resolve-expr.cc: Add "break rust" Easter egg

gcc/testsuite/ChangeLog:
	* lib/prune.exp (prune_ices):
	Also prune "You have broken GCC Rust. This is a feature."
	* rust/compile/break-rust1.rs: New test
	* rust/compile/break-rust2.rs: New test
	* rust/compile/break-rust3.rs: New test

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>