[ClangImporter] Add missing end-iterator check in Clang diag emission (swiftlang#15006)

This caused non-deterministic crashes when a high number of modules
containing diagnostics were compiled for the first time. The 9-entry
test case results in a crash on 64-bit macOS when run under Guard
Malloc without this change.

rdar://problem/33515925

Author: jrose-apple

lib/ClangImporter/ClangDiagnosticConsumer.cpp

@@ -165,8 +165,10 @@ SourceLoc ClangDiagnosticConsumer::resolveSourceLocation(
                                   const clang::SourceManager *toInsert) {
     return std::less<const clang::SourceManager *>()(inArray.get(), toInsert);
   });
-  if (iter->get() != &clangSrcMgr)
+  if (iter == sourceManagersWithDiagnostics.end() ||
+      iter->get() != &clangSrcMgr) {
     sourceManagersWithDiagnostics.insert(iter, &clangSrcMgr);
+  }
 
   return loc;
 }

test/ClangImporter/Inputs/custom-modules/Warnings1.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings1() {
+  old();
+}

test/ClangImporter/Inputs/custom-modules/Warnings2.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings2() {
+  old();
+}

test/ClangImporter/Inputs/custom-modules/Warnings3.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings3() {
+  old();
+}

test/ClangImporter/Inputs/custom-modules/Warnings4.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings4() {
+  old();
+}

test/ClangImporter/Inputs/custom-modules/Warnings5.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings5() {
+  old();
+}

test/ClangImporter/Inputs/custom-modules/Warnings6.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings6() {
+  old();
+}

test/ClangImporter/Inputs/custom-modules/Warnings7.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings7() {
+  old();
+}

test/ClangImporter/Inputs/custom-modules/Warnings8.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings8() {
+  old();
+}

test/ClangImporter/Inputs/custom-modules/Warnings9.h

@@ -0,0 +1,4 @@
+extern void old() __attribute__((deprecated));
+static void warnings9() {
+  old();
+}