DeclPrinter: add terse output mode and lots of tests

Add a flag PrintingPolicy::DontRecurseInDeclContext to provide "terse" output
from DeclPrinter.  The motivation is to use DeclPrinter to print declarations
in user-friendly format, without overwhelming user with inner detail of the
declaration being printed.

Also add many tests for DeclPrinter.  There are quite a few things that we
print incorrectly: search for WRONG in DeclPrinterTest.cpp -- and these tests
check our output against incorrect output, so that we can fix/refactor/rewrite
the DeclPrinter later.

llvm-svn: 162245