Cross-module-optimization: issue an error if TBD file generation is requested with CMO

This combination is not supported and may result in undefined-symbol linker errors.

rdar://problem/63238999

Author: eeckstein

include/swift/AST/DiagnosticsFrontend.def

@@ -234,6 +234,10 @@ WARNING(tbd_only_supported_in_whole_module,none,
         "TBD generation is only supported when the whole module can be seen",
         ())
 
+ERROR(tbd_not_supported_with_cmo,none,
+        "Test-Based InstallAPI (TBD) is not support with cross-module-optimization",
+        ())
+
 WARNING(linker_directives_choice_confusion,none,
         "only one of -emit-ldadd-cfile-path and -module-installname-map-file can be specified;"
         "the c file won't be generated",

lib/FrontendTool/FrontendTool.cpp

@@ -1004,6 +1004,12 @@ static bool writeTBDIfNeeded(CompilerInstance &Instance) {
     return false;
   }
 
+  if (Invocation.getSILOptions().CrossModuleOptimization) {
+    Instance.getDiags().diagnose(SourceLoc(),
+                                 diag::tbd_not_supported_with_cmo);
+    return false;
+  }
+
   const std::string &TBDPath = Invocation.getTBDPathForWholeModule();
 
   return writeTBD(Instance.getMainModule(), TBDPath, tbdOpts);
@@ -1404,7 +1410,7 @@ static bool validateTBDIfNeeded(const CompilerInvocation &Invocation,
       return false;
     }
 
-    // Cross-module optimization does not yet support TBD validation.
+    // Cross-module optimization does not support TBD.
     if (Invocation.getSILOptions().CrossModuleOptimization) {
       return false;
     }

test/Frontend/no-cmo-with-tbd.swift

@@ -0,0 +1,5 @@
+// RUN: not %target-swift-frontend -O -parse-as-library -cross-module-optimization -emit-tbd -module-name=Test %s -c -o /dev/null -verify 2>&1 | %FileCheck %s
+
+// CHECK: Test-Based InstallAPI (TBD) is not support with cross-module-optimization
+
+func foo() { }