[Orc] Try to turn Orc execution unit tests back on for Linux.
[Orc] Try to turn Orc execution unit tests back on for Linux.

The fix in r258324 (plus r258354) should allow Orc execution tests to run on
Linux.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@258358 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 8b5686809776b59de0d2ff06adaf12582b7175eb)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm