[clang][modules] Add a c23 module feature

[iana]

Summary

Add a c23 module feature for requires.

[iana]

Would we want to back port this to llvm 17?

[iana]

remove the extra #endif (which oddly didn't produce an error locally so I must've ran the tests wrong the first time...)

[v.g.vassilev]

LGTM!

In D159018#4622598, @iana wrote:

Would we want to back port this to llvm 17?

Yes probably.

[aaron.ballman]

In D159018#4623880, @v.g.vassilev wrote:

LGTM!

In D159018#4622598, @iana wrote:

Would we want to back port this to llvm 17?

Yes probably.

I believe we're only backporting critical fixes or fixes for regressions at this point, so I don't think we should try to backport this one right now.

That said, the changes LGTM!

clang/docs/Modules.rst

c99
   C99 support is available.
 
 c11
   C11 support is available.
 
 c17
   C17 support is available.
 
+c23
+  C23 support is available.
+
 freestanding
   A freestanding environment is available.
 
 gnuinlineasm
   GNU inline ASM is available.
 
 objc
   Objective-C support is available.

clang/docs/ReleaseNotes.rst

- ``structs``, ``unions``, and ``arrays`` that are const may now be used as
   GCC.
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 - Clang now accepts ``-std=c23`` and ``-std=gnu23`` as language standard modes,
   and the ``__STDC_VERSION__`` macro now expands to ``202311L`` instead of its
   previous placeholder value. Clang continues to accept ``-std=c2x`` and
   ``-std=gnu2x`` as aliases for C23 and GNU C23, respectively.
+- Clang now supports `requires c23` for module maps.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
 New Compiler Flags
 ------------------
 
 Deprecated Compiler Flags

clang/lib/Basic/Module.cpp

bool HasFeature = llvm::StringSwitch<bool>(Feature)
                         .Case("cplusplus14", LangOpts.CPlusPlus14)
                         .Case("cplusplus17", LangOpts.CPlusPlus17)
                         .Case("cplusplus20", LangOpts.CPlusPlus20)
                         .Case("cplusplus23", LangOpts.CPlusPlus23)
                         .Case("cplusplus26", LangOpts.CPlusPlus26)
                         .Case("c99", LangOpts.C99)
                         .Case("c11", LangOpts.C11)
                         .Case("c17", LangOpts.C17)
+                        .Case("c23", LangOpts.C23)
                         .Case("freestanding", LangOpts.Freestanding)
                         .Case("gnuinlineasm", LangOpts.GNUAsm)
                         .Case("objc", LangOpts.ObjC)
                         .Case("objc_arc", LangOpts.ObjCAutoRefCount)
                         .Case("opencl", LangOpts.OpenCL)
                         .Case("tls", Target.isTLSSupported())
                         .Case("zvector", LangOpts.ZVector)
                         .Default(Target.hasFeature(Feature) ||

clang/test/Modules/Inputs/DependsOnModule.framework/module.map

explicit module C99 {
     requires c99
   }
   explicit module C11 {
     requires c11
   }
   explicit module C17 {
     requires c17
   }
+  explicit module C23 {
+    requires c23
+  }
 }

clang/test/Modules/requires.m

 @import DependsOnModule.CXX26; // expected-note {{module imported here}}
 #else
 // [e-mail]/module.map:58 {{module 'DependsOnModule.C99' requires feature 'c99'}}
 @import DependsOnModule.C99; // expected-note {{module imported here}}
 // [e-mail]/module.map:61 {{module 'DependsOnModule.C11' requires feature 'c11'}}
 @import DependsOnModule.C11; // expected-note {{module imported here}}
 // [e-mail]/module.map:64 {{module 'DependsOnModule.C17' requires feature 'c17'}}
 @import DependsOnModule.C17; // expected-note {{module imported here}}
+// [e-mail]/module.map:67 {{module 'DependsOnModule.C23' requires feature 'c23'}}
+@import DependsOnModule.C23; // expected-note {{module imported here}}
 #endif