Fix yet another parser infinite loop

This commit is an example of fixing a common parser error: infinite
loop due to error recovery.

This error typically happens when we parse a list of items and fail to
parse a specific item at the current position.

One choices is to skip a token and try to parse a list item at the
next position. This is a good, but not universal, default. When
parsing a list of arguments in a function call, you, for example,
don't want to skip over `fn`, because it's most likely that it is a
function declaration, and not a mistyped arg:

```
fn foo() {
    quux(1, 2

fn bar() {
}
```

Another choice is to bail out of the loop immediately, but it isn't
perfect either: sometimes skipping over garbage helps:

```
quux(1, foo:, 92) // should skip over `:`, b/c that's part of `foo::bar`
```

In general, parser tries to balance these two cases, though we don't
have a definitive strategy yet.

However, if the parser accidentally neither skips over a token, nor
breaks out of the loop, then it becomes stuck in the loop infinitely
(there's an internal counter to self-check this situation and panic
though), and that's exactly what is demonstrated by the test.

To fix such situation, first of all, add the test case to tests/data/parser/{err,fuzz-failures}.

Then, run

```
RUST_BACKTRACE=short cargo test --package libsyntax2
````

to verify that parser indeed panics, and to get an idea what grammar
production is the culprit (look for `_list` functions!).

In this case, I see

```
  10: libsyntax2::grammar::expressions::atom::match_arm_list
             at crates/libsyntax2/src/grammar/expressions/atom.rs:309
```

and that's look like it might be a culprit. I verify it by adding
`eprintln!("loopy {:?}", p.current());` and indeed I see that this is
printed repeatedly.

Diagnosing this a bit shows that the problem is that
`pattern::pattern` function does not consume anything if the next
token is `let`. That is a good default to make cases like

```
let
let foo = 92;
```

where the user hasn't typed the pattern yet, to parse in a reasonable
they correctly.

For match arms, pretty much the single thing we expect is a pattern,
so, for a fix, I introduce a special variant of pattern that does not
do recovery.

crates/libsyntax2/src/grammar/expressions/atom.rs

@@ -323,11 +323,9 @@ fn match_arm_list(p: &mut Parser) {
 // }
 fn match_arm(p: &mut Parser) -> BlockLike {
     let m = p.start();
-    loop {
+    patterns::pattern_r(p, TokenSet::EMPTY);
+    while p.eat(PIPE) {
         patterns::pattern(p);
-        if !p.eat(PIPE) {
-            break;
-        }
     }
     if p.eat(IF_KW) {
         expr_no_struct(p);