Unify ui test inline error annotation style

[WaffleLapkin]

Proposal

Currently there are four styles of inline error annotations accepted by our testing infra:

//~ error lowercase, no colon
//~ error: lowercase, colon
//~ ERROR uppercase, no colon
//~ ERROR: uppercase, colon

i.e. you can either use lowercase or uppercase and either include colon or not.

Having multuple styles is not only unnecessary, but also slightly bad. It makes it harder to search for the annotations, it makes contributors (especially new ones) doubt which style to use, it makes it harder to read tests (since you need to recognize different styles).

As @jieyouxu said here, there are multiple places where compiletest is too lax in what it accepts for no real benefit. This is one of them. Others should probably be also addressed.

We should decide on a single style and enforce it. As always with styling decisions different people have different preferences, however enforcing any style would be better than the status quo.

Some data points:

• @petrochenkov notes that ERROR (uppercase, no colon) is the most popular style.
• miri and clippy are using ui_test which only accepts ERROR: (uppercase, colon).

Mentors or Reviewers

No mentorship is needed really, since this is a simple change, even if big.

Process

The main points of the Major Change Process are as follows:

• [x] File an issue describing the proposal.
• [ ] A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• [ ] Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

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[rustbot]

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cc @rust-lang/compiler

[apiraino]

Closing MCP for lack of a clear buyout and agreement on its form, see MCP process.

If the topic of discussion in this MCP should be reignited, feel free to reopen this proposal or a new one.