tests: reorganize tests so that they actually use the target platform

Most tests were using %swift or similar substitutions, which did not
include the target triple and SDK.  The driver was defaulting to the
host OS.  Thus, we could not run the tests when the standard library was
not built for OS X.

Swift SVN r24504

test/DebugInfo/alloca-init.swift

@@ -1,4 +1,5 @@
-// RUN: %swift -target x86_64-apple-macosx10.9 %s -emit-ir -g -o - | FileCheck %s
+// RUN: %target-swift-frontend %s -emit-ir -g -o - | FileCheck %s
+
 class A {
   var a : A?
   // CHECK: define {{.*}}1AcfMS0_FT_S0_