[Orc] Simplify LogicalDylib and move it back inside CompileOnDemandLayer. Also

[Orc] Simplify LogicalDylib and move it back inside CompileOnDemandLayer. Also
switch to using one indirect stub manager per logical dylib rather than one per
input module.

LogicalDylib is a helper class used by the CompileOnDemandLayer to manage
symbol resolution between modules during lazy compilation. In particular, it
ensures that internal symbols resolve correctly even in the case where multiple
input modules contain the same internal symbol name (which must to be promoted
to external hidden linkage so that functions in any given module can be split
out by lazy compilation). LogicalDylib's resolution scheme (before this commit)
required one stub-manager per input module. This made recompilation of functions
(by adding a module containing a new definition) difficult, as the stub manager
for any given symbol was bound to the module that supplied the original
definition. By using one stubs manager for the whole logical dylib symbols can
be more easily replaced, although support for doing this is not included in this
patch (it will be implemented in a follow up).

llvm-svn: 279952