[clang] [clang] add `-fimplicit-constexpr` flag (PR #136436)

[Hana Dusíková via cfe-commits]

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@@ -779,6 +779,9 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
 
   // TODO: Final number?
   Builder.defineMacro("__cpp_type_aware_allocators", "202500L");
+
+  if (LangOpts.ImplicitConstexpr) // same value as GCC
+    Builder.defineMacro("__cpp_implicit_constexpr", "20211111");
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hanickadot wrote:

this is to mirror GCC's behavior, but I'm happy to remove it

https://github.com/llvm/llvm-project/pull/136436