ModuleInterface: remark potential version differences between SDK and prebuilt modules

Prebuilt-module directory now contains a SystemVersion.plist file copied from the SDK
it's built from. This patch teaches the compiler to remark this version and the SDK version
when -Rmodule-interface-rebuild is specified. The difference between these versions could
help us debug unusable prebuilt modules.

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -286,10 +286,18 @@ struct ModuleRebuildInfo {
   /// Emits a diagnostic for all out-of-date compiled or forwarding modules
   /// encountered while trying to load a module.
   void diagnose(ASTContext &ctx, SourceLoc loc, StringRef moduleName,
-                 StringRef interfacePath) {
+                 StringRef interfacePath, StringRef prebuiltCacheDir) {
     ctx.Diags.diagnose(loc, diag::rebuilding_module_from_interface,
                        moduleName, interfacePath);
-
+    auto SDKVer = getSDKBuildVersion(ctx.SearchPathOpts.SDKPath);
+    llvm::SmallString<64> buffer = prebuiltCacheDir;
+    llvm::sys::path::append(buffer, "SystemVersion.plist");
+    auto PBMVer = getSDKBuildVersionFromPlist(buffer.str());
+    if (!SDKVer.empty() && !PBMVer.empty()) {
+      // Remark the potential version difference.
+      ctx.Diags.diagnose(loc, diag::sdk_version_pbm_version, SDKVer,
+                         PBMVer);
+    }
     // We may have found multiple failing modules, that failed for different
     // reasons. Emit a note for each of them.
     for (auto &mod : outOfDateModules) {
@@ -911,7 +919,7 @@ class ModuleInterfaceLoaderImpl {
     // Diagnose that we didn't find a loadable module, if we were asked to.
     auto remarkRebuild = [&]() {
       rebuildInfo.diagnose(ctx, diagnosticLoc, moduleName,
-                           interfacePath);
+                           interfacePath, prebuiltCacheDir);
     };
     // If we found an out-of-date .swiftmodule, we still want to add it as
     // a dependency of the .swiftinterface. That way if it's updated, but