[clang] [Serialization] [ASTWriter] Try to not record namespace as much as possible (PR #179178)

[Cameron Angus via cfe-commits]

kamrann wrote:

The following code (compiled with `-stdlib=libc++`)
```
struct S
{
	virtual ~S() = default;
};

import std;
using std::size_t;
```
now yields `error: use of undeclared identifier 'std'`. I've confirmed that reverting this commit corrects the error, so there is some regression here.

https://github.com/llvm/llvm-project/pull/179178