Fix clang-tidy warnings from CLion 2020.2 (#6387)

* Fix clang-tidy warnings in FieldValue

* Update clang-tidy exclusions for CLion 2020.2

Author: wilhuff

Firestore/core/.clang-tidy

@@ -17,7 +17,7 @@
 #   -modernize-use-equals-delete
 #     GoogleTest generates test classes that use the old idiom of making
 #     default constructors and operator= private.
-Checks:          'bugprone-*,cert-*,-cert-dcl50-cpp,-cert-err58-cpp,google-*,objc-*,readability-*,-readability-else-after-return,-readability-implicit-bool-conversion,misc-*,modernize-*,-modernize-use-equals-default,-modernize-use-equals-delete,clang-diagnostic-*,clang-analyzer-*'
+Checks:          'bugprone-*,cert-*,-cert-dcl50-cpp,-cert-err58-cpp,google-*,objc-*,readability-*,-readability-else-after-return,-readability-implicit-bool-conversion,misc-*,modernize-*,-modernize-use-equals-default,-modernize-use-equals-delete,-modernize-use-trailing-return-type,clang-diagnostic-*,clang-analyzer-*'
 WarningsAsErrors: ''
 HeaderFilterRegex: ''
 CheckOptions:

Firestore/core/src/model/field_value.h

@@ -84,7 +84,7 @@ class FieldValue {
 
   FieldValue();
 
-  FieldValue(ObjectValue object);  // NOLINT(runtime/explicit)
+  FieldValue(const ObjectValue& object);  // NOLINT(runtime/explicit)
 
   /** Returns the true type for this value. */
   Type type() const {
@@ -409,7 +409,7 @@ class FieldValue::ServerTimestamp {
 };
 
 // Pretend you can automatically upcast from ObjectValue to FieldValue.
-inline FieldValue::FieldValue(ObjectValue object)
+inline FieldValue::FieldValue(const ObjectValue& object)
     : FieldValue(object.AsFieldValue()) {
 }