Add descriptions to completed expressions

Summary:
Completing inside the expression command now uses the new description API
to also provide additional information to the user. For now this information
are the types of variables/fields and the signatures of completed function calls.

Reviewers: #lldb, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: JDevlieghere, lldb-commits

Differential Revision: https://reviews.llvm.org/D52103

llvm-svn: 342385