[llvm][Demangle] Add getter for FunctionEncoding::Attrs

[Michael137]

Summary

In LLDB we use the Itanium mangle tree API to compare
function encodings of mangled names when selecting a
candidate symbol for expression evaluation. We now
also include ABI tags in this comparison. This patch
makes this possible by exposing the Attrs in the mangle
tree API.

[Michael137]

• Rebase

[Michael137]

@urnathan @ldionne any idea who is responsible for mirroring this change in libcxxabi?

[ldionne]

In D131330#3706912, @Michael137 wrote:

@urnathan @ldionne any idea who is responsible for mirroring this change in libcxxabi?

It's actually the other way around! According to libcxxabi/src/demangle/README.txt, the change should be done in libcxxabi/src/demangle and then libcxxabi/src/demangle/cp-to-llvm.sh can be used to mirror. Note that changing libcxxabi will also have the effect of triggering libc++abi's CI pipeline, which in turn should have at least some coverage for the demangler changes.

[Michael137]

• Sync with libcxxabi

libcxxabi/src/demangle/ItaniumDemangle.h

public:
   template<typename Fn> void match(Fn F) const {
     F(Ret, Name, Params, Attrs, CVQuals, RefQual);
   }
 
   Qualifiers getCVQuals() const { return CVQuals; }
   FunctionRefQual getRefQual() const { return RefQual; }
   NodeArray getParams() const { return Params; }
   const Node *getReturnType() const { return Ret; }
+  const Node *getAttrs() const { return Attrs; }
 
   bool hasRHSComponentSlow(OutputBuffer &) const override { return true; }
   bool hasFunctionSlow(OutputBuffer &) const override { return true; }
 
   const Node *getName() const { return Name; }
 
   void printLeft(OutputBuffer &OB) const override {
     if (Ret) {

llvm/include/llvm/Demangle/ItaniumDemangle.h

public:
   template<typename Fn> void match(Fn F) const {
     F(Ret, Name, Params, Attrs, CVQuals, RefQual);
   }
 
   Qualifiers getCVQuals() const { return CVQuals; }
   FunctionRefQual getRefQual() const { return RefQual; }
   NodeArray getParams() const { return Params; }
   const Node *getReturnType() const { return Ret; }
+  const Node *getAttrs() const { return Attrs; }
 
   bool hasRHSComponentSlow(OutputBuffer &) const override { return true; }
   bool hasFunctionSlow(OutputBuffer &) const override { return true; }
 
   const Node *getName() const { return Name; }
 
   void printLeft(OutputBuffer &OB) const override {
     if (Ret) {