rustc: use Name numbers rather than the Show impl for constants.

Using the Show impl for Names created global symbols with names like
`"str\"str\"(1027)"`. This adjusts strings, binaries and vtables to
avoid using that impl.

Fixes #15799.
auto merge of #16213 : huonw/rust/more-token-numbers, r=pnkfelix

Using the Show impl for Names created global symbols with names like
`"str\"str\"(1027)"`. This adjusts strings, binaries and vtables to
avoid using that impl.