src: use stricter compile-time guidance

SnapshotSerializerDeserializer::GetName() appears to confuse static
analysis such as Coverity.

This changes the function structure to a sequence of if-else blocks and
marks all branch conditions as constexpr. (Unfortunately, this results
in a dangling 'else' keyword in the V macro.)

As per a request in the PR discussion, this change does _not_ ensure
that GetName<T>() can only be called for known types T and instead still
returns an empty string in that case.

Also use std::is_unsigned_v instead of !std::is_signed_v.

src/node_snapshotable.cc

@@ -164,22 +164,19 @@ class SnapshotSerializerDeserializer {
   V(std::string)
 
 #define V(TypeName)                                                            \
-  if (std::is_same_v<T, TypeName>) {                                           \
+  if constexpr (std::is_same_v<T, TypeName>) {                                 \
     return #TypeName;                                                          \
-  }
+  } else  // NOLINT(readability/braces)
     TYPE_LIST(V)
 #undef V
 
-    std::string name;
-    if (std::is_arithmetic_v<T>) {
-      if (!std::is_signed_v<T>) {
-        name += "u";
-      }
-      name += std::is_integral_v<T> ? "int" : "float";
-      name += std::to_string(sizeof(T) * 8);
-      name += "_t";
+    if constexpr (std::is_arithmetic_v<T>) {
+      return (std::is_unsigned_v<T>   ? "uint"
+              : std::is_integral_v<T> ? "int"
+                                      : "float") +
+             std::to_string(sizeof(T) * 8) + "_t";
     }
-    return name;
+    return "";
   }
 
   bool is_debug = false;