[CodeGen][ObjC] Remove the leading `l_` from ObjC symbols and make

private symbols in the __DATA segment internal.

This prevents the linker from removing the symbol names. Keeping the
symbols visible enables tools to collect various information about the
symbols, for example, tools that discover whether or not a symbol gets
dirtied.

rdar://problem/48887111

Differential Revision: https://reviews.llvm.org/D61454

llvm-svn: 360359