InstrProf: Optionally generate coverage maps when profiling

This adds the -profile-coverage-mapping option to swift, and teaches
SILGenProfiling to generate mappings from source ranges to counters.

Swift SVN r25266

test/SILGen/coverage_while.swift

@@ -0,0 +1,41 @@
+// RUN: %target-swift-frontend -profile-generate -profile-coverage-mapping -emit-sorted-sil -emit-sil -module-name coverage_while %s | FileCheck %s
+
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_while.foo
+func foo() -> Int32 {
+  var x : Int32 = 0
+  // CHECK: [[@LINE+1]]:9 -> [[@LINE+1]]:17 : (0 + 1)
+  while (x < 10) {
+    x++
+  }
+
+  // CHECK: [[@LINE+1]]:9 -> [[@LINE+1]]:18 : (0 + 2)
+  while (--x > 0) {
+    if (x % 2 == 0) { continue }
+  }
+
+  // CHECK: [[@LINE+1]]:9 -> [[@LINE+1]]:18 : ((0 + 4) - 5)
+  while (x < 100) {
+    if (x++ == 10) { break }
+  }
+
+  // CHECK: [[@LINE+1]]:9 -> [[@LINE+1]]:18 : ((0 + 6) - 9)
+  while (x < 100) {
+    x++
+    while (true) { break }
+    if (x % 2 == 0) { continue }
+    // CHECK: [[@LINE-1]]:33 -> [[@LINE+2]]:4 : (6 - 8)
+    if (x > 30) { break }
+  }
+
+  // CHECK: [[@LINE+1]]:9 -> [[@LINE+1]]:18 : ((0 + 10) - 11)
+  while (x < 100) {
+    if (x == 40) { // CHECK: [[@LINE]]:18 -> [[@LINE+2]]:6 : 11
+      return x
+    }
+    ++x
+  }
+  // CHECK: [[@LINE-1]]:4 -> [[@LINE+1]]:11 : (0 - 11)
+  return x
+}
+
+foo()