Rollup merge of #145055 - bjorn3:move_metadata_symbol_export, r=saethlin

Move metadata symbol export from exported_non_generic_symbols to exported_symbols

The metadata symbol must not be encoded in the crate metadata, and must be exported from proc-macros. Handling the export of the metadata symbol in exported_symbols handles both things at once without requiring manual fixups elsewhere.

Author: Zalathar