[clang] [clang][ASTImporter] Fix AST import if anonymous namespaces are merged (PR #128735)

[Balázs Kéri via cfe-commits]

balazske wrote:

I am now not really sure what happens, if there is a single anonymous namespace for a TU or there are separate ones for the anonymous namespaces in for example `extern "C"` blocks (and how it should work according to the C++ standard). Probably adding more tests where there is a name conflict will show what happens. The ideal solution looks like to separate the imported anonymous namespace from the existing one but this may be difficult to implement.

https://github.com/llvm/llvm-project/pull/128735