Expose an API to print a group of decls (like "int a,b;").

Expose an API to print a group of decls (like "int a,b;").
Make StmtPrinter use DeclPrinter to print all declarations.  Merge
declarations in the limited case of an unnamed TagDecl followed by one
or more declarations using that TagDecl directly.  Change
SuppressTypeSpecifiers to the more general SuppressSpecifiers, and
use it to suppress stuff like "typedef" and "extern".  Replace
OwnedTag with SuppressTag, since it's more convenient to print
declarations from DeclPrinter at the moment.
improvements to declaration printing.  Fix pretty-printing for K&R
function definitions and __builtin_va_arg.

We're now to the point where the pretty-printing output for non-trivial
programs can actually be piped back into clang.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72608 91177308-0d34-0410-b5e6-96231b3b80d8