Use two-argument static_assert

One-argument `static_assert` became valid only in C++17, so it’s disallowed by http://go/cstyle#C++_Version. Switch to two-argument `static_assert`.

PiperOrigin-RevId: 253274513

Author: Googler

remote_config/src/remote_config_ios.mm

@@ -44,9 +44,9 @@
     kValueSourceDefaultValue,  // FIRRemoteConfigSourceDefault
     kValueSourceStaticValue,   // FIRRemoteConfigSourceStatic
 };
-static_assert(FIRRemoteConfigSourceRemote == 0);
-static_assert(FIRRemoteConfigSourceDefault == 1);
-static_assert(FIRRemoteConfigSourceStatic == 2);
+static_assert(FIRRemoteConfigSourceRemote == 0, "FIRRemoteConfigSourceRemote is not 0");
+static_assert(FIRRemoteConfigSourceDefault == 1, "FIRRemoteConfigSourceDefault is not 1");
+static_assert(FIRRemoteConfigSourceStatic == 2, "FIRRemoteConfigSourceStatic is not 2");
 
 // Regular expressions used to determine if the config value is a "valid" bool.
 // Written to match what is used internally by the Java implementation.