[analyzer] Add --force-analyze-debug-code option to scan-build

to force debug build and hopefully enable more precise warnings.

Static Analyzer is much more efficient when built in debug mode
(-UNDEBUG) so we advice users to enable it manually. This may be
inconvenient in case of large complex projects (think about Linux
distros e.g. Android or Tizen). This patch adds a flag to scan-build
which inserts -UNDEBUG automatically.

Differential Revision: http://reviews.llvm.org/D16200

llvm-svn: 261204