[clang] [clang-tools-extra] [clangd] [AST] Handle uninitialized type constraints (PR #110496)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

@falbrechtskirchinger 

We need to resort to `-fallow-pch-with-compiler-errors` :)

Specifically, add the following file to `clang/test/PCH` (or somehow merge it into a pre-existing test, which I'd prefer)

Name it with a reasonable name e.g. `cxx2a-invalid-constraint-serialization.cpp`

```cpp
// RUN: %clang_cc1 -std=c++2a -emit-pch -fallow-pch-with-compiler-errors %s -o %t
// RUN: %clang_cc1 -std=c++2a -include-pch %t -verify %s

namespace GH99036 {

template <typename T>
concept C;

template <C U> void f();

} // namespace GH99036

```

You can see a crash without your patch.

https://github.com/llvm/llvm-project/pull/110496