Merge pull request #398 from DougGregor/link-swiftrt

[CMake] Make sure to link swiftrt.o when building the Swift Dispatch overlay.

Signed-off-by: Kim Topley <ktopley@apple.com>

Author: DougGregor

PATCHES

@@ -436,3 +436,4 @@ github commits starting with 29bdc2f from
 [6162a1d] APPLIED rdar://44568645
 [c55ff6f] APPLIED rdar://54572081
 [c4a7149] APPLIED rdar://54572081
+[edce1fe] APPLIED rdar://54572081