Lint on FileCheck directives with missing colon

In current compiletest -> FileCheck setup (e.g. codegen/assembly tests), if you forgor a colon : in one of the FileCheck directives, it gets silently ignored by both compilest and FileCheck, meaning that you aren't testing anything.

// tests/assembly/selftest.rs

//@ assembly-output: emit-asm

#![crate_type = "lib"]

// CHECK-LABEL: foo:
//-- CHECK-LABEL: bar: # <- make sure this test is actually testing
// CHECK-NOT ret       # <- notice the missing `:`?
#[no_mangle]
pub fn foo(x: u8) {}

This test currently will just silently pass, even though we wanted to check ret doesn't exist (made-up example for illustration purposes). Realized while reviewing https://github.com/rust-lang/rust/pull/128018.

Example missing colon (real example): https://github.com/rust-lang/rust/pull/125626.

We should probably aim to lint on cases like this.

Start with the compiletest-to-FileCheck setup and the example in tests/assembly/selftest.rs, then reproduce how a directive without a colon is handled. Trace where FileCheck directives are parsed and determine how the missing-colon case should be reported. Done means the malformed CHECK-NOT example no longer passes silently and a regression test covers the lint.