Revert "Don't count inreg params when mangling fastcall functions"

This reverts commit r214981.

I'm not sure what I was thinking when I wrote this. Testing with MSVC
shows that this function is mangled to '@f@8':
  int __fastcall f(int a, int b);

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220492 91177308-0d34-0410-b5e6-96231b3b80d8
Make this test a bit stricter.

This now:
* Forces the linker to include the internal definition.
* Checks the full output.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220494 91177308-0d34-0410-b5e6-96231b3b80d8