[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++.

---------

Co-authored-by: Balazs Benics <benicsbalazs@gmail.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 29f9cf3..aadda69 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -623,6 +623,9 @@ static bool FixupInvocation(CompilerInvocation &Invocation,
     LangOpts.RawStringLiterals = true;
   }
 
+  LangOpts.NamedLoops =
+      Args.hasFlag(OPT_fnamed_loops, OPT_fno_named_loops, LangOpts.C2y);
+
   // Prevent the user from specifying both -fsycl-is-device and -fsycl-is-host.
   if (LangOpts.SYCLIsDevice && LangOpts.SYCLIsHost)
     Diags.Report(diag::err_drv_argument_not_allowed_with) << "-fsycl-is-device"