Swift: port frontend-invocations test to linux

`swift-frontend` will be in `$PATH` on Linux, and apparently it does
not require the `-sdk` option.

Author: redsun82

swift/integration-tests/osx-only/frontend-invocations/Makefile

@@ -0,0 +1,15 @@
+OS=$(shell uname)
+ifeq ($(OS),Darwin)
+SDK=-sdk $(shell xcrun -show-sdk-path)
+FRONTEND=$(shell xcrun -find swift-frontend)
+else
+SDK=""
+FRONTEND=swift-frontend
+endif
+
+all:
+	$(FRONTEND) -frontend -c A.swift $(SDK)
+	$(FRONTEND) -frontend -c B.swift -o B.o $(SDK)
+	$(FRONTEND) -frontend -c -primary-file C.swift $(SDK)
+	$(FRONTEND) -frontend -c -primary-file D.swift -o D.o $(SDK)
+	$(FRONTEND) -frontend -c -primary-file E.swift Esup.swift -o E.o $(SDK)