Refactor ClangUserExpression::Parse [NFC]

Summary:
This patch splits out functionality from the `Parse` method into different methods.
This benefits the code completion work (which should reuse those methods) and makes the
code a bit more readable.

Note that this patch is as minimal as possible. Some of the code in the new methods definitely
needs more refactoring.

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D48339

llvm-svn: 336734