[clang] [Clang] Allow all address spaces to be converted to the default (PR #112248)

[Shilei Tian via cfe-commits]

shiltian wrote:

I think the default AS is expected to be losslessly converted to other AS but not the other way around, though I understand it is not clearly stated in LangRef or other places and a lot of code just assumes it can.

https://github.com/llvm/llvm-project/pull/112248