[ORC] Do not include non-global symbols in getObjectSymbolFlags.

Private symbols are not visible outside the object file, and so not defined by
the object file from ORC's perspective.

No test case yet. Ideally this would be a unit test parsing a checked-in binary,
but I am not aware of any way to reference the LLVM source root from a unit
test.

llvm-svn: 340703