Move diagnostic kind name to the end of the diagnostic. (#4437)

Also surround it in square brackets rather than parentheses. This
matches the format used by Clang and GCC, and means diagnostics will
still match the `file:line:col: error: ` pattern used by some IDE tools.

Before:
```console
fail_builtins.carbon:11:11: error(AliasRequiresNameRef): alias initializer must be a name reference
```

After:
```console
fail_builtins.carbon:11:11: error: alias initializer must be a name reference [AliasRequiresNameRef]
```

Also tighten up test regex to only match on `STDERR` lines that list a
file name.

toolchain/parse/testdata/match/fail_missing_default_statements_block.carbon

@@ -11,7 +11,7 @@
 fn f() -> i32 {
   match (3) {
     default =>
-  // CHECK:STDERR: fail_missing_default_statements_block.carbon:[[@LINE+3]]:3: error(ExpectedMatchCaseBlock): expected `{` after `=>`
+  // CHECK:STDERR: fail_missing_default_statements_block.carbon:[[@LINE+3]]:3: error: expected `{` after `=>` [ExpectedMatchCaseBlock]
   // CHECK:STDERR:   }
   // CHECK:STDERR:   ^
   }