Merge #648 #649

648: Fix issue with mangled name on function items with substitutions r=philberty a=philberty

Rust legacy name mangling does not contain the substitutions as part of
its mangled name for Item's. Rust avoids duplicate symbol collisions with
the legacy mangling scheme, but including a 128bit hash at the end of the
symbol, which is made up with metadata and in this case the mangled symbol
contains a hash of the type which this function is.

Fixes #647


649: Get rid of get_locus_slow r=philberty a=dkm

From Mark Wielaard: https://gcc.gnu.org/pipermail/gcc-rust/2021-August/000154.html

> 
> In various places there was the following hack:
> 
>   /* HACK: slow way of getting location from base expression through
>      virtual methods. */
>   virtual Location get_locus_slow () const { return Location (); }
> 
> The problem with get_locus_slow () is that if a subclass didn't
> override it then there was no real location. get_locus_slow was
> missing for Module, ExternCrate, UseDeclaration, Function, TypeAlias,
> StructStruct, TupleStruct, Enum, Union, ConstantItem, StaticItem,
> Trait, ImplBlock, ExternBlock, EmptyStmt, ExprStmtWithoutBlock and
> ExprStmtWithBlock. All do have a get_locus () function.
> 
> Simply replace the get_locus_slow virtual method with a real virtual
> Location get_locus () const = 0 method so we know if something
> really doesn't have a location. This was only the case for
> MacroRulesDefinition.

Co-authored-by: Philip Herron <philip.herron@embecosm.com>
Co-authored-by: Mark Wielaard <mark@klomp.org>