[clang-tidy] Don't ignore default set of checks when a config file is found.

[alexfh]

Summary

This makes clang-tidy merge the default set of checks with the one
provided in the configuration file instead of just using the checks from the
config file. This adds a way to modify the default set of checks while the
previous behavior required to always define the set of checks completely.

[djasper]

Looks good.

clang-tidy/ClangTidyOptions.cpp

llvm::ErrorOr<ClangTidyOptions> ParsedOptions =
         ConfigHandler.second((*Text)->getBuffer());
     if (!ParsedOptions) {
       if (ParsedOptions.getError())
         llvm::errs() << "Error parsing " << ConfigFile << ": "
                      << ParsedOptions.getError().message() << "\n";
       continue;
     }
 
-    ClangTidyOptions Defaults = DefaultOptionsProvider::getOptions(Directory);
-    // Only use checks from the config file.
-    Defaults.Checks = None;
-    return Defaults.mergeWith(*ParsedOptions).mergeWith(OverrideOptions);
+    return DefaultOptionsProvider::getOptions(Directory)
+        .mergeWith(*ParsedOptions)
+        .mergeWith(OverrideOptions);
   }
   return llvm::None;
 }
 
 /// \brief Parses -line-filter option and stores it to the \c Options.
 std::error_code parseLineFilter(StringRef LineFilter,
                                 clang::tidy::ClangTidyGlobalOptions &Options) {
   llvm::yaml::Input Input(LineFilter);

test/clang-tidy/config-files.cpp

 // RUN: clang-tidy -dump-config %S/Inputs/config-files/- -- | FileCheck %s -check-prefix=CHECK-BASE
-// CHECK-BASE: Checks: from-parent
+// CHECK-BASE: Checks: {{.*}}from-parent
 // CHECK-BASE: HeaderFilterRegex: parent
 // RUN: clang-tidy -dump-config %S/Inputs/config-files/1/- -- | FileCheck %s -check-prefix=CHECK-CHILD1
-// CHECK-CHILD1: Checks: from-child1
+// CHECK-CHILD1: Checks: {{.*}}from-child1
 // CHECK-CHILD1: HeaderFilterRegex: child1
 // RUN: clang-tidy -dump-config %S/Inputs/config-files/2/- -- | FileCheck %s -check-prefix=CHECK-CHILD2
-// CHECK-CHILD2: Checks: from-parent
+// CHECK-CHILD2: Checks: {{.*}}from-parent
 // CHECK-CHILD2: HeaderFilterRegex: parent
 // RUN: clang-tidy -dump-config -checks='from-command-line' -header-filter='from command line' %S/Inputs/config-files/- -- | FileCheck %s -check-prefix=CHECK-COMMAND-LINE
-// CHECK-COMMAND-LINE: Checks: from-parent,from-command-line
+// CHECK-COMMAND-LINE: Checks: {{.*}}from-parent,from-command-line
 // CHECK-COMMAND-LINE: HeaderFilterRegex: from command line