Improve completion experience for headers

Summary: When calling `completing-read', we should provide a default to prevent the behavior described in https://github.com/DarwinAwardWinner/ido-completing-read-plus#why-does-ret-sometimes-not-select-the-first-completion-on-the-list--why-is-there-an-empty-entry-at-the-beginning-of-the-completion-list--what-happened-to-old-style-default-selection.  Also, don't use an assertion to check whether the user selected a header; raise a proper signal instead.

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43969

llvm-svn: 329566