Mark the default constructor of Matcher<> explicit.

This prevents implicitly constructing a matcher from {}.

include/gmock/gmock-matchers.h

@@ -321,7 +321,7 @@ class Matcher : public internal::MatcherBase<T> {
   // Constructs a null matcher.  Needed for storing Matcher objects in STL
   // containers.  A default-constructed matcher is not yet initialized.  You
   // cannot use it until a valid value has been assigned to it.
-  Matcher() {}
+  explicit Matcher() {}  // NOLINT
 
   // Constructs a matcher from its implementation.
   explicit Matcher(const MatcherInterface<T>* impl)