Extend 'target symbols add' to load symbols from a given module

Now -shlib flag can be provided alongside with names of symbols files:

(lldb) target symbols add --shlib stripper-lib.so unstripper-lib.so

This is helpful when default matching mechanisms by name and UUID
can't find a module, and the user needs to explicitly specify
which module the given symbol file belongs to.

Differential Revision: https://reviews.llvm.org/D35607

llvm-svn: 308933