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

[via cfe-commits]

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@@ -0,0 +1,51 @@
+// RUN: %clang_cc1 -verify=normal,both -std=c++23 %s
+// RUN: %clang_cc1 -verify=implicit,both -fimplicit-constexpr -std=c++23 %s
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Sirraide wrote:

We should also test at least C++17 mode and see how this interacts with features that are not allowed in `constexpr` functions in that language mode (i.e. destructors, operator `new`, `goto`).

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