arm64ec incorrect mangling instantiating C++ template

[efriedma-quic]

template <typename X> struct Wrapper {
  int GetValue(void) const;
};
struct A {
  int GetValue(void) const;
};
template<typename X> int Wrapper<X>::GetValue(void) const { return 3; }
template class Wrapper<A>;

clang produces ?GetValue@$$h?$Wrapper@UA@@@@QEBAHXZ, MSVC produces ?GetValue@?$Wrapper@UA@@@@$$hQEBAHXZ. I guess the algorithm in llvm::getArm64ECMangledFunctionName is wrong.

CC @mstorsjo @cjacek @dpaoliello @steplong

[llvmbot]

@llvm/issue-subscribers-c-1

Author: Eli Friedman (efriedma-quic)

``` template <typename X> struct Wrapper { int GetValue(void) const; }; struct A { int GetValue(void) const; }; template<typename X> int Wrapper<X>::GetValue(void) const { return 3; } template class Wrapper<A>; ```

clang produces ?GetValue@$$h?$Wrapper@<!-- -->UA@@@@<!-- -->QEBAHXZ, MSVC produces ?GetValue@?$Wrapper@<!-- -->UA@@@@$$hQEBAHXZ. I guess the algorithm in llvm::getArm64ECMangledFunctionName is wrong.

CC @mstorsjo @cjacek @dpaoliello @steplong

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eli Friedman (efriedma-quic)

``` template <typename X> struct Wrapper { int GetValue(void) const; }; struct A { int GetValue(void) const; }; template<typename X> int Wrapper<X>::GetValue(void) const { return 3; } template class Wrapper<A>; ```

clang produces ?GetValue@$$h?$Wrapper@<!-- -->UA@@@@<!-- -->QEBAHXZ, MSVC produces ?GetValue@?$Wrapper@<!-- -->UA@@@@$$hQEBAHXZ. I guess the algorithm in llvm::getArm64ECMangledFunctionName is wrong.

CC @mstorsjo @cjacek @dpaoliello @steplong

[dpaoliello]

I believe that I have the correct algorithm - PR out shortly