Refactor the Args class.

There were a number of issues with the Args class preventing
efficient use of strings and incoporating LLVM's StringRef class.
The two biggest were:

1. Backing memory stored in a std::string, so we would frequently
   have to use const_cast to get a mutable buffer for passing to
   various low level APIs.
2. backing std::strings stored in a std::list, which doesn't
   provide random access.

I wanted to solve these two issues so that we could provide
StringRef access to the underlying arguments, and also a way
to provide range-based access to the underlying argument array
while still providing convenient c-style access via an argv style
const char**.

The solution here is to store arguments in a single "entry" class
which contains the backing memory, a StringRef with precomputed
length, and the quote char.  The backing memory is a manually
allocated const char* so that it is not invalidated when the
container is resized, and there is a separate argv array provided
for c-style access.

Differential revision: https://reviews.llvm.org/D25099

llvm-svn: 283157

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