Add the `lto_codegen_set_export_dynamic' function.

This function sets the `_exportDynamic' ivar. When that's set, we export all
symbols (e.g. we don't run the internalize pass). This is equivalent to the
`--export-dynamic' linker flag in GNU land:

--export-dynamic
  When creating a dynamically linked executable, add all symbols to the dynamic
  symbol table. The dynamic symbol table is the set of symbols which are visible
  from dynamic objects at run time. If you do not use this option, the dynamic
  symbol table will normally contain only those symbols which are referenced by
  some dynamic object mentioned in the link. If you use dlopen to load a dynamic
  object which needs to refer back to the symbols defined by the program, rather
  than some other dynamic object, then you will probably need to use this option
  when linking the program itself.

The Darwin linker will support this via the `-export_dynamic' flag. We should
modify clang to support this via the `-rdynamic' flag.

llvm-svn: 169656