rustc: make error messages containing generic more self-explanatory.

Unsuffixed literals like 1 and 1.1, and free type parameters sometimes
have to be printed in error messages, which ended up with <V0>, <VI0>
and <VF0>. This change puts the words "generic" and "integer"/"float"
into the message so it's not a completely black box.
auto merge of #11513 : huonw/rust/generic-errs, r=alexcrichton

Unsuffixed literals like 1 and 1.1, and free type parameters sometimes
have to be printed in error messages, which ended up with \<V0>, \<VI0>
and \<VF0>. This change puts the words "generic" and "integer"/"float"
into the message so it's not a completely black box.