[val] Fixup id name output (#2158)

This CL changes the id/name output from the validator to always use a
consistent id[%name] style. This removes the need for getIdOrName. The
name lookup is changed to use the NameMapper so the output is consistent
with what the disassembler will produce.

Fixes #2137

test/cpp_interface_test.cpp

@@ -58,7 +58,8 @@ TEST(CppInterface, SuccessfulRoundTrip) {
     EXPECT_EQ(0u, position.line);
     EXPECT_EQ(0u, position.column);
     EXPECT_EQ(1u, position.index);
-    EXPECT_STREQ("ID 1 has not been defined\n  %2 = OpSizeOf %1 %3\n", message);
+    EXPECT_STREQ("ID 1[%1] has not been defined\n  %2 = OpSizeOf %1 %3\n",
+                 message);
   });
   EXPECT_FALSE(t.Validate(binary));