libsyntax/librustc: Allow mut qualifier in patterns.
libsyntax: Get rid of some logic for some obsolete syntax.
Test for mut in ident patterns.
librustc: Don't emit unused mut lint twice.
libsyntax: Get rid of obsolete obsolete warning.
auto merge of #10026 : luqmana/rust/mut-pat, r=pcwalton

Fixes #9792.