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

[via cfe-commits]

================
@@ -413,6 +413,7 @@ BENIGN_LANGOPT(ArrowDepth, 32, 256,
                "maximum number of operator->s to follow")
 BENIGN_LANGOPT(InstantiationDepth, 32, 1024,
                "maximum template instantiation depth")
+LANGOPT(ImplicitConstexpr, 1, 0, "evaluate everything as it is a constexpr enabled")
----------------
Sirraide wrote:

```suggestion
COMPATIBLE_LANGOPT(ImplicitConstexpr, 1, 0, "make functions implicitly 'constexpr'")
```
Since this only really takes effect when the AST is constructed, I don’t think this should cause any compatibilty problems.

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