Migrated static functions that print decls into a DeclPrinter class,

which is now used (or subclasssed) by the ASTConsumers.  This new class
stores a FILE* that is used for writing, instead of just hardwiring output
to stderr (it defaults to stderr if no FILE* is provided).

Modified CreateASTPrinter() to accept a FILE* for printing.

llvm-svn: 44377