Avoid using string operator += for a single character (#7747)

This is flagged as a mistake by clang-tidy 24. String's operator `+=` is
pretty bad in general, this moves a few uses to push_back.

toolchain/sem_ir/inst_fingerprinter.cpp

@@ -150,12 +150,12 @@ class StringFingerprintStore {
     bool first = true;
     for (const auto& item : contents_) {
       if (!first) {
-        result += ",";
+        result.push_back(',');
       }
       first = false;
       result += item;
     }
-    result += "}";
+    result.push_back('}');
     return SaveString(std::move(result));
   }
 

toolchain/sem_ir/inst_namer.cpp

@@ -409,7 +409,7 @@ auto InstNamer::Namespace::AllocateName(
   }
 
   // Append numbers until we find an available name.
-  name += ".";
+  name.push_back('.');
   auto name_size_without_counter = name.size();
   for (int counter = 1;; ++counter) {
     name.resize(name_size_without_counter);