swift234

Simplify parsing and representation of extension declarations.

Now that we don't have generic parameter lists at arbitrary positions
within the extended type of an extension declaration, simplify the
representation of the extended type down to a TypeLoc along with a
(compiler-synthesized) generic parameter list.

On the parsing side, just parse a type for the extended type, rather
than having a special grammar. We still reject anything that is not a
nominal type (of course), but it's simpler just to call it a type.

As a drive-by, fix the crasher when extending a type with module
qualification, rdar://problem/20900870.

Swift SVN r28469