[clang-cl] Support /std:clatest (#147284)

cl.exe has /std:clatest, analogous to /std:c++latest, which sets the
language mode to the latest standard. For C, that's C23. This adds
support for the option.

Fixes #147233

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -249,6 +249,8 @@ C Language Changes
 - The ``[[clang::assume()]]`` attribute is now correctly recognized in C. The
   ``__attribute__((assume()))`` form has always been supported, so the fix is
   specific to the attribute syntax used.
+- The ``clang-cl`` driver now recognizes ``/std:clatest`` and sets the language
+  mode to C23. (#GH147233)
 
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7212,6 +7212,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
       LanguageStandard = llvm::StringSwitch<StringRef>(StdArg->getValue())
                              .Case("c11", "-std=c11")
                              .Case("c17", "-std=c17")
+                             // TODO: add c23 when MSVC supports it.
+                             .Case("clatest", "-std=c23")
                              .Default("");
       if (LanguageStandard.empty())
         D.Diag(clang::diag::warn_drv_unused_argument)

clang/test/Driver/cl-options.c

@@ -362,6 +362,12 @@
 // RUN: %clang_cl -c -### /std:c11 -- %s 2>&1 | FileCheck -check-prefix CHECK-C11 %s
 // CHECK-C11: -std=c11
 
+// RUN: %clang_cl -c -### /std:c17 -- %s 2>&1 | FileCheck -check-prefix CHECK-C17 %s
+// CHECK-C17: -std=c17
+
+// RUN: %clang_cl -c -### /std:clatest -- %s 2>&1 | FileCheck -check-prefix CHECK-CLATEST %s
+// CHECK-CLATEST: -std=c23
+
 // For some warning ids, we can map from MSVC warning to Clang warning.
 // RUN: %clang_cl -wd4005 -wd4100 -wd4910 -wd4996 -wd12345678 -### -- %s 2>&1 | FileCheck -check-prefix=Wno %s
 // Wno: "-cc1"