go.tools/oracle: set AllASTs=true always, and simplify.
go.tools/oracle: set AllASTs=true always, and simplify.

Every one of the oracle's query modes needs to have typed ASTs
available, at least transiently, so that the -pos flag can be
interpreted.  (The only mode that doesn't need the -pos flag
is callgraph, but that needs PTA.)  So we hard-code it to true.

This change fixes a bug in the 'implements' query that causes
-pos parsing to fail.  (This wasn't exposed by the tests
because they are degenerate in that the query always occurs in
the main package, which is specified ad-hoc, i.e. as a source
file not an import path.  That's unfortunate, but this
change renders the distinction uninteresting in future.)

R=crawshaw, dominik.honnef
CC=golang-dev
https://codereview.appspot.com/13334050

skyportsystems/go.tools,webfd/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,8limbs/gotools,skyportsystems/go.tools,d0f/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools