IR: Add Function metadata attachments

Add IR support for `Metadata` attachments.  Assembly and bitcode support
will follow shortly, but for now we just have unit tests.  This is part
of PR23340.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235783 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Function.cpp

@@ -324,6 +324,9 @@ void Function::dropAllReferences() {
   // Prefix and prologue data are stored in a side table.
   setPrefixData(nullptr);
   setPrologueData(nullptr);
+
+  // Metadata is stored in a side-table.
+  clearMetadata();
 }
 
 void Function::addAttribute(unsigned i, Attribute::AttrKind attr) {