[ORC] Promote and rename private symbols inside the CompileOnDemand layer,

rather than require them to have been promoted before being passed in.

Dropping this precondition is better for layer composition (CompileOnDemandLayer
was the only one that placed pre-conditions on the modules that could be added).
It also means that the promoted private symbols do not show up in the target
JITDylib's symbol table. Instead, they are confined to the hidden implementation
dylib that contains the actual definitions.

For the 403.gcc testcase this cut down the public symbol table size from ~15,000
symbols to ~4000, substantially reducing symbol dependence tracking costs.

llvm-svn: 344078