[PATCH] Add -fno-modules-implicit-builds.

[Richard Smith]

Please add test coverage showing that we can use explicitly-specified modules via `-fmodule-file=`, and that we can't use implicitly-build modules that are in the cache.


================
Comment at: lib/Frontend/CompilerInstance.cpp:1363-1367
@@ -1362,2 +1362,7 @@
     Known = KnownModules.insert(std::make_pair(Path[0].first, Module)).first;
+  } else if (!getLangOpts().ImplicitModules) {
+    getDiagnostics().Report(ModuleNameLoc, diag::err_module_build_disabled)
+        << ModuleName;
+    ModuleBuildFailed = true;
+    return ModuleLoadResult();
   } else {
----------------
This will also reject modules loaded via `-fmodule-file=`.

================
Comment at: lib/Frontend/CompilerInstance.cpp:1380
@@ -1374,3 +1379,3 @@
     auto Override = ModuleFileOverrides.find(ModuleName);
     bool Explicit = Override != ModuleFileOverrides.end();
 
----------------
The check should go somewhere around here, after we've determined whether the user gave us a file containing this module. `if (!Explicit && !ImplicitModules)`.

http://reviews.llvm.org/D6707

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