IR: Add assembly/bitcode support for function metadata attachments

Add serialization support for function metadata attachments (added in
r235783).  The syntax is:

    define @foo() !attach !0 {

Metadata attachments are only allowed on functions with bodies.  Since
they come before the `{`, they're not really part of the body; since
they require a body, they're not really part of the header.  In
`LLParser` I gave them a separate function called from `ParseDefine()`,
`ParseOptionalFunctionMetadata()`.

In bitcode, I'm using the same `METADATA_ATTACHMENT` record used by
instructions.  Instruction metadata attachments are included in a
special "attachment" block at the end of a `Function`.  The attachment
records are laid out like this:

    InstID (KindID MetadataID)+

Note that these records always have an odd number of fields.  The new
code takes advantage of this to recognize function attachments (which
don't need an instruction ID):

    (KindID MetadataID)+

This means we can use the same attachment block already used for
instructions.

This is part of PR23340.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235785 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/AsmWriter.cpp

@@ -776,12 +776,12 @@ void SlotTracker::processFunction() {
     if (!BB.hasName())
       CreateFunctionSlot(&BB);
 
+    processFunctionMetadata(*TheFunction);
+
     for (auto &I : BB) {
       if (!I.getType()->isVoidTy() && !I.hasName())
         CreateFunctionSlot(&I);
 
-      processInstructionMetadata(I);
-
       // We allow direct calls to any llvm.foo function here, because the
       // target may not be linked into the optimizer.
       if (const CallInst *CI = dyn_cast<CallInst>(&I)) {
@@ -804,9 +804,15 @@ void SlotTracker::processFunction() {
 }
 
 void SlotTracker::processFunctionMetadata(const Function &F) {
-  for (auto &BB : F)
+  SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
+  for (auto &BB : F) {
+    F.getAllMetadata(MDs);
+    for (auto &MD : MDs)
+      CreateMetadataSlot(MD.second);
+
     for (auto &I : BB)
       processInstructionMetadata(I);
+  }
 }
 
 void SlotTracker::processInstructionMetadata(const Instruction &I) {
@@ -2541,6 +2547,10 @@ void AssemblyWriter::printFunction(const Function *F) {
     writeOperand(F->getPrologueData(), true);
   }
 
+  SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
+  F->getAllMetadata(MDs);
+  printMetadataAttachments(MDs, " ");
+
   if (F->isDeclaration()) {
     Out << '\n';
   } else {