[libc++][module] Fixes std::string UDL. (#75000)

The fix changes the way the validation script determines the qualified
name of a declaration. Inline namespaces without a reserved name are now
always part of the name. The Clang code only does this when the names
are ambigious. This method is often used for the operator""foo for UDLs.

Adjusted the newly flagged issue and removed a work-around in the test
code that is no longer required.

Fixes https://github.com/llvm/llvm-project/issues/72427