Disable performance-inefficient-string-concatenation

Although it probably increases the performance, the fix has horrible syntax.
Either you write a lot += on multiple lines, or you chain .append(). Chaining
append is however a bit bothersome, because you need create an explicit copy for
the first string.

.clang-tidy

@@ -72,6 +72,7 @@ Checks:
   - -modernize-use-trailing-return-type
   - -modernize-use-using
   - -performance-enum-size
+  - -performance-inefficient-string-concatenation
   - -readability-avoid-const-params-in-decls
   - -readability-avoid-nested-conditional-operator
   - -readability-braces-around-statements