Provide operator<< for stream output of DeclarationNames

ASTDumper was already trying to do this & instead got an implicit bool
conversion by surprise (thus printing out 0 or 1 instead of the name of
the declaration). To avoid that issue & simplify call sites, simply make
it the normal/expected operator<<(raw_ostream&, ...) overload & simplify
all the existing call sites. (bonus: this function doesn't need to be a
member or friend, it's just using public API in DeclarationName)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181832 91177308-0d34-0410-b5e6-96231b3b80d8