[Clang] [C2y] Implement N3355 ‘Named Loops’ (#152870)
This implements support for [named
loops](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3355.htm) for
C2y.
When parsing a `LabelStmt`, we create the `LabeDecl` early before we parse
the substatement; this label is then passed down to `ParseWhileStatement()`
and friends, which then store it in the loop’s (or switch statement’s) `Scope`;
when we encounter a `break/continue` statement, we perform a lookup for
the label (and error if it doesn’t exist), and then walk the scope stack and
check if there is a scope whose preceding label is the target label, which
identifies the jump target.
The feature is only supported in C2y mode, though a cc1-only option
exists for testing (`-fnamed-loops`), which is mostly intended to try
and make sure that we don’t have to refactor this entire implementation
when/if we start supporting it in C++.
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Co-authored-by: Balazs Benics <benicsbalazs@gmail.com>
