IR: Default the Metadata::dump() argument "harder" after r232275

Use an overload instead of a default argument for `Metadata::dump()`.
The latter seems to require calling `dump(nullptr)` explicitly when
using a debugger, where as the former doesn't.

Other than utility for debugging, there's NFC here.

llvm-svn: 232315
diff --git a/llvm/lib/IR/AsmWriter.cpp b/llvm/lib/IR/AsmWriter.cpp
index 509fb42..ea68220 100644
--- a/llvm/lib/IR/AsmWriter.cpp
+++ b/llvm/lib/IR/AsmWriter.cpp
@@ -3290,6 +3290,9 @@
 void NamedMDNode::dump() const { print(dbgs()); }
 
 LLVM_DUMP_METHOD
+void Metadata::dump() const { dump(nullptr); }
+
+LLVM_DUMP_METHOD
 void Metadata::dump(const Module *M) const {
   print(dbgs(), M);
   dbgs() << '\n';