librustc: Fix the issue with labels shadowing variable names by making
the leading quote part of the identifier for the purposes of hygiene.

This adopts @jbclements' solution to #14539.

I'm not sure if this is a breaking change or not.

Closes #12512.

[breaking-change]
auto merge of #14797 : pcwalton/rust/label-namespace, r=alexcrichton

the leading quote part of the identifier for the purposes of hygiene.

This adopts @jbclements' solution to #12512.

I'm not sure if this is a breaking change or not.

[breaking-change]

r? @alexcrichton