[ORC] Add (partial) weak symbol support to the CompileOnDemand layer.

This adds partial support for weak functions to the CompileOnDemandLayer by
modifying the addLogicalModule method to check for existing stub definitions
before building a new stub for a weak function. This scheme is sufficient to
support ODR definitions, but fails for general weak definitions if strong
definition is encountered after the first weak definition. (A more extensive
refactor will be required to fully support weak symbols).

This patch does *not* add weak symbol support to RuntimeDyld: I hope to add
that in the near future.

llvm-svn: 277896