[Backtracing] Add ImageMap instead of just using an Array.

We want to be able to efficiently serialise lists of images, and to do so
it makes most sense to create a separate `ImageMap` type.  This also provides
a useful place to put methods to e.g. find an image by address or by build
ID.

rdar://124913332

test/Backtracing/ImageMap.swift

@@ -0,0 +1,27 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -parse-as-library -Onone -o %t/ImageMap
+// RUN: %target-codesign %t/ImageMap
+// RUN: %target-run %t/ImageMap | %FileCheck %s
+
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+// REQUIRES: executable_test
+// REQUIRES: backtracing
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+import Runtime
+
+@main
+struct ImageMapTest {
+  static func main() {
+    let map = ImageMap.capture()
+
+    // We expect ImageMap, followed by one or more additional lines
+
+    // CHECK: {{0x[0-9a-f]*-0x[0-9a-f]*}} {{(<no build ID>|[0-9a-f]*)}} ImageMap {{.*}}/ImageMap
+    // CHECK-NEXT: {{0x[0-9a-f]*-0x[0-9a-f]*}} {{(<no build ID>|[0-9a-f]*)}} [[NAME:[^ ]*]] {{.*}}/[[NAME]]
+    print(map)
+  }
+}
+
+