[clang] [Modules] Record whether VarDecl initializers contain side effects (PR #143739)

[Jonas Devlieghere via cfe-commits]

JDevlieghere wrote:

This breaks the std module import feature in LLDB: https://green.lab.llvm.org/job/llvm.org/view/LLDB/job/as-lldb-cmake/28186/. Also reproduces in pre-commit CI: https://github.com/llvm/llvm-project/actions/runs/15830795745/job/44622706379?pr=145366

We're hitting the following assert:

```
Assertion failed: (!isNull() && "Cannot retrieve a NULL type pointer"), function getCommonPtr, file Type.h, line 952.
```


https://github.com/llvm/llvm-project/pull/143739