[clang] [coroutine] Create coroutine body in the correct eval context (PR #78589)

[via cfe-commits]

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@@ -804,6 +804,9 @@ Bug Fixes in This Version
   Objective-C++ property accesses to not be converted to a function call
   to the getter in the placement-args of new-expressions.
   Fixes (`#65053 <https://github.com/llvm/llvm-project/issues/65053>`_)
+- Fix an issue with missing symbol definitions when the first coroutine
+  statement is part of an disabled branch of ``if constexpr``.
----------------
cor3ntin wrote:

```suggestion
  statement appears in a discarded ``if constexpr`` branch.
```

https://github.com/llvm/llvm-project/pull/78589