[core] Fix google-explicit-constructor errors

As reported by clang-tidy-8.

platform/default/src/mbgl/storage/local_file_source.cpp

@@ -19,7 +19,7 @@ namespace mbgl {
 
 class LocalFileSource::Impl {
 public:
-    Impl(const ActorRef<Impl>&) {}
+    explicit Impl(const ActorRef<Impl>&) {}
 
     void request(const std::string& url, const ActorRef<FileSourceRequest>& req) {
         if (!acceptsURL(url)) {

platform/default/src/mbgl/storage/sqlite3.cpp

@@ -46,9 +46,7 @@ void setTempPath(const std::string& path) {
 
 class DatabaseImpl {
 public:
-    DatabaseImpl(sqlite3* db_)
-        : db(db_)
-    {
+    explicit DatabaseImpl(sqlite3* db_) : db(db_) {
         const int error = sqlite3_extended_result_codes(db, true);
         if (error != SQLITE_OK) {
             mbgl::Log::Warning(mbgl::Event::Database, error, "Failed to enable extended result codes: %s", sqlite3_errmsg(db));

platform/default/src/mbgl/util/async_task.cpp

@@ -12,10 +12,7 @@ namespace util {
 
 class AsyncTask::Impl {
 public:
-    Impl(std::function<void()>&& fn)
-        : async(new uv_async_t),
-          task(std::move(fn)) {
-
+    explicit Impl(std::function<void()> fn) : async(new uv_async_t), task(std::move(fn)) {
         auto* loop = reinterpret_cast<uv_loop_t*>(RunLoop::getLoopHandle());
         if (uv_async_init(loop, async, asyncCallback) != 0) {
             throw std::runtime_error("Failed to initialize async.");

platform/default/src/mbgl/util/jpeg_reader.cpp

@@ -79,8 +79,7 @@ static void on_error_message(j_common_ptr cinfo) {
 }
 
 struct jpeg_info_guard {
-    jpeg_info_guard(jpeg_decompress_struct* cinfo)
-        : i_(cinfo) {}
+    explicit jpeg_info_guard(jpeg_decompress_struct* cinfo) : i_(cinfo) {}
 
     ~jpeg_info_guard() {
         jpeg_destroy_decompress(i_);

platform/glfw/glfw_gl_backend.cpp

@@ -7,8 +7,7 @@
 
 class GLFWGLRenderableResource final : public mbgl::gl::RenderableResource {
 public:
-    GLFWGLRenderableResource(GLFWGLBackend& backend_) : backend(backend_) {
-    }
+    explicit GLFWGLRenderableResource(GLFWGLBackend& backend_) : backend(backend_) {}
 
     void bind() override {
         backend.setFramebufferBinding(0);

platform/glfw/test_writer.cpp

@@ -20,7 +20,7 @@ public:
 
 class SetCamera final : public TestOperationSerializer {
 public:
-    SetCamera(mbgl::CameraOptions camera_) : camera(std::move(camera_)) {}
+    explicit SetCamera(mbgl::CameraOptions camera_) : camera(std::move(camera_)) {}
 
     void serialize(Writer& writer) const override {
         if (camera.zoom) {
@@ -59,7 +59,7 @@ private:
 
 class SetStyle final : public TestOperationSerializer {
 public:
-    SetStyle(const mbgl::style::Style& style) : url(style.getURL()) {}
+    explicit SetStyle(const mbgl::style::Style& style) : url(style.getURL()) {}
 
     void serialize(Writer& writer) const override {
         writer.StartArray();
@@ -74,7 +74,7 @@ private:
 
 class SetInitialSize final : public TestOperationSerializer {
 public:
-    SetInitialSize(const mbgl::Size& size) : width(size.width), height(size.height) {}
+    explicit SetInitialSize(const mbgl::Size& size) : width(size.width), height(size.height) {}
 
     void serialize(Writer& writer) const override {
         writer.Key("width");

platform/linux/src/headless_backend_glx.cpp

@@ -17,7 +17,7 @@ private:
     struct Key { explicit Key() = default; };
 
 public:
-    GLXDisplayConfig(Key) {
+    explicit GLXDisplayConfig(Key) {
         if (!XInitThreads()) {
             throw std::runtime_error("Failed to XInitThreads.");
         }

src/mbgl/annotation/annotation_tile.cpp

@@ -70,8 +70,7 @@ const GeometryCollection& AnnotationTileFeature::getGeometries() const {
 
 class AnnotationTileLayerData {
 public:
-    AnnotationTileLayerData(std::string name_) : name(std::move(name_)) {
-    }
+    explicit AnnotationTileLayerData(std::string name_) : name(std::move(name_)) {}
 
     const std::string name;
     std::vector<std::shared_ptr<const AnnotationTileFeatureData>> features;

src/mbgl/style/expression/compound_expression.cpp

@@ -33,7 +33,7 @@ bool operator==(const VarargsType& lhs, const VarargsType& rhs) {
 template <typename T>
 struct Varargs : std::vector<T> {
     template <class... Args>
-    Varargs(Args&&... args) : std::vector<T>(std::forward<Args>(args)...) {}
+    explicit Varargs(Args&&... args) : std::vector<T>(std::forward<Args>(args)...) {}
 };
 
 namespace detail {

src/mbgl/style/expression/expression.cpp

@@ -12,7 +12,7 @@ public:
     const Feature& feature;
     mutable optional<GeometryCollection> geometry;
 
-    GeoJSONFeature(const Feature& feature_) : feature(feature_) {}
+    explicit GeoJSONFeature(const Feature& feature_) : feature(feature_) {}
     GeoJSONFeature(const Feature& feature_, const CanonicalTileID& canonical) : feature(feature_) {
         geometry = convertGeometry(feature.geometry, canonical);
         // https://github.com/mapbox/geojson-vt-cpp/issues/44