Fix immutable string concats with ints `BuildConcatenatedImmutableString("a", 10, "b")` would construct "a\nb" because the implementation lacked int overloads and ints would be promoted to chars implicitly. Fix by implementing simpler way to calculate digits for numbers and then add useful overloads. Remove ImmutableStringBuilder::appendDecimal() since the code is already expecting << for all other types, and the bug was due to this expectation. Bug: angleproject:376417347 Change-Id: Iea5e14a6e2fede068b704754b9203db794dd5bf0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5972641 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Kimmo Kinnunen <kkinnunen@apple.com>