When building a module, keep *all* declared methods in the global method pool.

As an optimization, we only kept declared methods with distinct
signatures in the global method pool, to keep the method lists
small. Under modules, however, one could have two different methods
with the same signature that occur in different (sub)modules. If only
the later submodule is important, message sends to 'id' with that
selector would fail because the first method (the only one that got
into the method pool) was hidden. When building a module, keep *all*
of the declared methods.

I did a quick check of both module build time and uses of modules, and
found no performance regression despite this causing us to keep more
methods in the global method pool. Fixes <rdar://problem/14148896>.

llvm-svn: 184504