[swift-ide-test] Emit a diagnostic if we can't find the module to print.

Fixes rdar://problem/41048812.

Author: DougGregor

test/IDE/print_module_missing.swift

@@ -0,0 +1,11 @@
+// Make sure we emit some kind of error message when we cannot find the module
+// to print.
+
+// RUN: not %target-swift-ide-test -print-module -print-interface -no-empty-line-between-members -module-to-print=NoSuchModule -source-filename=%s 2> %t.err
+// RUN: %FileCheck %s -check-prefix=CHECK-MISSING < %t.err
+
+// RUN: not %target-swift-ide-test -print-module -print-interface -no-empty-line-between-members -module-to-print=Swift.NoSuchSubModule -source-filename=%s 2> %t.suberr
+// RUN: %FileCheck %s -check-prefix=CHECK-MISSING-SUBMODULE < %t.suberr
+
+// CHECK-MISSING: 'NoSuchModule'
+// CHECK-MISSING-SUBMODULE: 'Swift.NoSuchSubModule'

tools/swift-ide-test/swift-ide-test.cpp

@@ -1854,6 +1854,8 @@ static int doPrintModules(const CompilerInvocation &InitInvok,
     // Get the (sub)module to print.
     auto *M = getModuleByFullName(Context, ModuleToPrint);
     if (!M) {
+      llvm::errs() << "error: could not find module '" << ModuleToPrint
+                   << "'\n";
       ExitCode = 1;
       continue;
     }
@@ -1873,6 +1875,8 @@ static int doPrintModules(const CompilerInvocation &InitInvok,
     if (ModuleName.size() > 1) {
       M = getModuleByFullName(Context, ModuleName[0]);
       if (!M) {
+        llvm::errs() << "error: could not find module '" << ModuleName[0]
+                     << "'\n";
         ExitCode = 1;
         continue;
       }