[5.7][Test] Help CoreGraphics_test.swift defeat the optimizer.

This test calls `CGPoint.applying` and `CGSize.applying` and ignores the return value, then checks for the call in the resulting IR. The optimizer appears to have become clever enough to notice that these calls can be eliminated entirely because their result is unused, causing the test to fail.

Instead of `let _ =`, use a `blackHole` function to make these values be used.

rdar://99489134
(cherry picked from commit e275f40)

Author: mikeash

test/ClangImporter/CoreGraphics_test.swift

@@ -6,6 +6,9 @@ import CoreGraphics
 // REQUIRES: OS=macosx
 // REQUIRES: CPU=x86_64
 
+@_silgen_name("blackHole")
+func blackHole<T>(_ value: T) -> Void
+
 // CHECK: [[SWITCHTABLE:@.*]] = private unnamed_addr constant [8 x i64] [i64 0, i64 12, i64 23, i64 34, i64 45, i64 55, i64 67, i64 71]
 
 // CHECK-LABEL: define swiftcc i64 {{.*}}testEnums{{.*}} {
@@ -99,9 +102,9 @@ public func testRenames(transform: CGAffineTransform, context: CGContext,
 // CHECK:   call void @CGAffineTransformInvert(%struct.CGAffineTransform* {{.*}}, %struct.CGAffineTransform* {{.*}})
 // CHECK:   call void @CGAffineTransformConcat(%struct.CGAffineTransform* {{.*}}, %struct.CGAffineTransform* {{.*}}, %struct.CGAffineTransform* {{.*}})
 
-  let _ = point.applying(transform)
+  blackHole(point.applying(transform))
   var rect = rect.applying(transform)
-  let _ = size.applying(transform)
+  blackHole(size.applying(transform))
 // CHECK:   %{{.*}} = {{(tail )?}}call { double, double } @CGPointApplyAffineTransform(double %{{.*}}, double %{{.*}}, %struct.CGAffineTransform* {{.*}})
 // CHECK:   call void @CGRectApplyAffineTransform(%struct.CGRect* {{.*}}, %struct.CGRect* {{.*}}, %struct.CGAffineTransform* {{.*}})
 // CHECK:   %{{.*}} = {{(tail )?}}call { double, double } @CGSizeApplyAffineTransform(double %{{.*}}, double %{{.*}}, %struct.CGAffineTransform* {{.*}})