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
Decouple dllexport/dllimport from linkage

Representing dllexport/dllimport as distinct linkage types prevents using
these attributes on templates and inline functions.

Instead of introducing further mixed linkage types to include linkonce and
weak ODR, the old import/export linkage types are replaced with a new
separate visibility-like specifier:

  define available_externally dllimport void @f() {}
  @Var = dllexport global i32 1, align 4

Linkage for dllexported globals and functions is now equal to their linkage
without dllexport. Imported globals and functions must be either
declarations with external linkage, or definitions with
AvailableExternallyLinkage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199204 91177308-0d34-0410-b5e6-96231b3b80d8