scan-build: Add an option to show the description in the list of defect

Summary:
This patch adds an option //--show-description// to add the defect description to the list of defect. This helps to get a better understanding of the defect without opening the page.

For example, this is used for Firefox:
https://people.mozilla.org/~sledru/reports/fx-scan-build/

Reviewers: rizsotto.mailinglist, zaks.anna

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D22810

llvm-svn: 277328