Introduce Parser::completeExpression.

This function returns a list of completions for a given expression and
completion position.

Differential Revision: http://llvm-reviews.chandlerc.com/D2261

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200497 91177308-0d34-0410-b5e6-96231b3b80d8