[Modules] Start making explicit modules produce deterministic output.

There are two aspects of non-determinism fixed here, which was the
minimum required to cause at least an empty module to be deterministic.

First, the random number signature is only inserted into the module when
we are building modules implicitly. The use case for these random
signatures is to work around the very fact that modules are not
deterministic in their output when working with the implicitly built and
populated module cache. Eventually this should go away entirely when
we're confident that Clang is producing deterministic output.

Second, the on-disk hash table is populated based on the order of
iteration over a DenseMap. Instead, use a MapVector so that we can walk
it in insertion order.

I've added a test that an empty module, when built twice, produces the
same binary PCM file.

llvm-svn: 233115

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