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

[Hana Dusíková via cfe-commits]

================
@@ -6612,6 +6612,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.AddLastArg(CmdArgs, options::OPT_fconstexpr_depth_EQ);
   Args.AddLastArg(CmdArgs, options::OPT_fconstexpr_steps_EQ);
 
+  Args.AddLastArg(CmdArgs, options::OPT_fimplicit_constexpr);
----------------
hanickadot wrote:

I have added check for c++, but it would be goot to enable for 14+ only (but not sure how to do it in this file, will look at it tomorrow)

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