Move Args::StringTo*** functions to a new OptionArgParser class

Summary:
The idea behind this is to move the functionality which depend on other lldb
classes into a separate class. This way, the Args class can be turned
into a lightweight arc+argv wrapper and moved into the lower lldb
layers.

Reviewers: jingham, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D44306

llvm-svn: 329677