Fix fastcall mangling of dllimported symbols

fastcall requires @ as global prefix instead of _ but getNameWithPrefix
wrongly assumes the OutName buffer is empty and replaces at index 0.
For imported functions this buffer is pre-filled with "__imp_" resulting
in broken "@_imp_foo@0" mangling.

Instead replace at the proper index. We also never have to prepend the
@-prefix because this fastcall mangling is only used on 32-bit Windows
targets which have _ has global prefix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199203 91177308-0d34-0410-b5e6-96231b3b80d8