Modify ParseArgs to return the InputArgList by value - there's no need for...

Modify ParseArgs to return the InputArgList by value - there's no need for dynamic allocation/ownership here

The one caller that does anything other than keep this variable on the
stack is the single use of DerivedArgList in Clang, which is a bit more
interesting but can probably be cleaned up/simplified a bit further
(have DerivedArgList take ownership of the InputArgList rather than
needing to reference its Args indirectly) which I'll try to after this.

llvm-svn: 240345