[clang] [Clang] Instantiate local constexpr functions eagerly (PR #95660)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

Linux CI failed with an unrelated error.
```
TIMEOUT: lldb-api :: functionalities/fork/concurrent_vfork/TestConcurrentVFork.py (87750 of 87750)
  | ******************** TEST 'lldb-api :: functionalities/fork/concurrent_vfork/TestConcurrentVFork.py' FAILED ********************
```

https://github.com/llvm/llvm-project/pull/95660