[clang][Basic] Add helper APIs to get language version codes from LangOptions (#163348)

Motivated by this discussion:
https://github.com/llvm/llvm-project/pull/163208#discussion_r2426842999

We will soon want to emit language version codes into debug-info.
Instead of replicating the `LangOptions -> version code` mapping we
thought we'd try to share some of the logic with the Clang frontend.

This patch teaches `LangStandard` about language versions (currently just C++ and C).

1 files changed, 4 insertions, 35 deletions

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index b899fb9c6494..baad63179d89 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -459,43 +459,12 @@ static void InitializeStandardPredefinedMacros(const TargetInfo &TI,
   //      value is, are implementation-defined.
   // (Removed in C++20.)
   if (!LangOpts.CPlusPlus) {
-    if (LangOpts.C2y)
-      Builder.defineMacro("__STDC_VERSION__", "202400L");
-    else if (LangOpts.C23)
-      Builder.defineMacro("__STDC_VERSION__", "202311L");
-    else if (LangOpts.C17)
-      Builder.defineMacro("__STDC_VERSION__", "201710L");
-    else if (LangOpts.C11)
-      Builder.defineMacro("__STDC_VERSION__", "201112L");
-    else if (LangOpts.C99)
-      Builder.defineMacro("__STDC_VERSION__", "199901L");
-    else if (!LangOpts.GNUMode && LangOpts.Digraphs)
-      Builder.defineMacro("__STDC_VERSION__", "199409L");
+    if (std::optional<uint32_t> Lang = LangOpts.getCLangStd())
+      Builder.defineMacro("__STDC_VERSION__", Twine(*Lang) + "L");
   } else {
     //   -- __cplusplus
-    if (LangOpts.CPlusPlus26)
-      // FIXME: Use correct value for C++26.
-      Builder.defineMacro("__cplusplus", "202400L");
-    else if (LangOpts.CPlusPlus23)
-      Builder.defineMacro("__cplusplus", "202302L");
-    //      [C++20] The integer literal 202002L.
-    else if (LangOpts.CPlusPlus20)
-      Builder.defineMacro("__cplusplus", "202002L");
-    //      [C++17] The integer literal 201703L.
-    else if (LangOpts.CPlusPlus17)
-      Builder.defineMacro("__cplusplus", "201703L");
-    //      [C++14] The name __cplusplus is defined to the value 201402L when
-    //      compiling a C++ translation unit.
-    else if (LangOpts.CPlusPlus14)
-      Builder.defineMacro("__cplusplus", "201402L");
-    //      [C++11] The name __cplusplus is defined to the value 201103L when
-    //      compiling a C++ translation unit.
-    else if (LangOpts.CPlusPlus11)
-      Builder.defineMacro("__cplusplus", "201103L");
-    //      [C++03] The name __cplusplus is defined to the value 199711L when
-    //      compiling a C++ translation unit.
-    else
-      Builder.defineMacro("__cplusplus", "199711L");
+    Builder.defineMacro("__cplusplus",
+                        Twine(*LangOpts.getCPlusPlusLangStd()) + "L");
 
     //   -- __STDCPP_DEFAULT_NEW_ALIGNMENT__
     //      [C++17] An integer literal of type std::size_t whose value is the