Remove some leftover changes related to the Swift calling convention.

Support for the Swift calling convention has now been added to LLVM trunk.
There were a few changes in the upstream-with-swift branch on github that
were related and are no longer needed. This change removes them.

apple-llvm-split-commit: 6da1b2710fd3befb75bce79aefd4fc3783dcf802
apple-llvm-split-dir: llvm/