[llvm-nm]Add support for --no-demangle

GNU nm has --no-demangle, so llvm-nm should too. It disables the
--demangle switch. The patch also allows --demangle to be specified
multiple times (the last of all --no-demangle/--demangle switches
takes precedence).

Reviewed by: grimar, rupprecht, mattd

Differential Revision: https://reviews.llvm.org/D60134

llvm-svn: 357575