[NFC] [C++20] [Modules] Rename ASTContext::getNamedModuleForCodeGen to...

[NFC] [C++20] [Modules] Rename ASTContext::getNamedModuleForCodeGen to ASTContext::getCurrentNamedModule

The original name "ASTContext::getNamedModuleForCodeGen" is not properly
reflecting the usage of the interface. This interface can be used to
judge the current module unit in both sema analysis and code generation.
So the original name was not so correct.