[tidy] Check modernize-use-auto

Ref: http://clang.llvm.org/extra/clang-tidy/checks/modernize-use-auto.html

src/mbgl/geometry/glyph_atlas.cpp

@@ -52,7 +52,7 @@ Rect<uint16_t> GlyphAtlas::addGlyph(uintptr_t tileUID,
     const uint8_t buffer = 3;
 
     std::map<uint32_t, GlyphValue>& face = index[fontStack];
-    std::map<uint32_t, GlyphValue>::iterator it = face.find(glyph.id);
+    auto it = face.find(glyph.id);
 
     // The glyph is already in this texture.
     if (it != face.end()) {

test/api/api_misuse.cpp

@@ -13,7 +13,7 @@
 using namespace mbgl;
 
 TEST(API, RenderWithoutCallback) {
-    FixtureLogObserver* log = new FixtureLogObserver();
+    auto log = new FixtureLogObserver();
     Log::setObserver(std::unique_ptr<Log::Observer>(log));
 
     util::RunLoop loop;

test/style/style_parser.cpp

@@ -26,7 +26,7 @@ TEST_P(StyleParserTest, ParseStyle) {
     ASSERT_FALSE(infoDoc.HasParseError());
     ASSERT_TRUE(infoDoc.IsObject());
 
-    FixtureLogObserver* observer = new FixtureLogObserver();
+    auto observer = new FixtureLogObserver();
     Log::setObserver(std::unique_ptr<Log::Observer>(observer));
 
     style::Parser parser;

test/util/thread_local.cpp

@@ -80,8 +80,8 @@ TEST(ThreadLocalStorage, AutoReclaim) {
 
     unsigned counter = 0;
 
-    DtorCounter* dtorCounter1 = new DtorCounter{ &counter };
-    DtorCounter* dtorCounter2 = new DtorCounter{ &counter };
+    auto dtorCounter1 = new DtorCounter{ &counter };
+    auto dtorCounter2 = new DtorCounter{ &counter };
 
     ThreadContext context = {"Test"};