Use export_as for autolinking frameworks

framework module SomeKitCore {
  ...
  export_as SomeKit
}

Given the module above, while generting autolink information during
codegen, clang should to emit '-framework SomeKitCore' only if SomeKit
was not imported in the relevant TU, otherwise it should use '-framework
SomeKit' instead.

rdar://problem/38269782

llvm-svn: 330152