Fix windows warning/error over unsigned int (facebook#34947)

Summary:
react-native-windows treats compiler c++ warning as errors, this is one of them. It fixes it by assigning i to unsigned instead of auto.

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[Internal] [Changed] - Fix react-native-windows compiler warning in AttributedString

Pull Request resolved: facebook#34947

Test Plan: [code](https://github.com/microsoft/react-native-windows/pull/10479/files#diff-c14f3a9383607a661977aa577c75bbc547827502950e0244721ef021f549fbe2) is already merged into react-native-windows and passing all tests. The for loop changed is also used directly above in AttributedString::compareTextAttributesWithoutFrame

Reviewed By: shwanton

Differential Revision: D40459186

Pulled By: NickGerleman

fbshipit-source-id: 848375f75f3df0cd086d758a239e86f39b43aa3f

Author: TatianaKapos

ReactCommon/react/renderer/attributedstring/AttributedString.cpp

@@ -112,7 +112,7 @@ bool AttributedString::compareTextAttributesWithoutFrame(
     return false;
   }
 
-  for (unsigned i = 0; i < fragments_.size(); i++) {
+  for (size_t i = 0; i < fragments_.size(); i++) {
     if (fragments_[i].textAttributes != rhs.fragments_[i].textAttributes ||
         fragments_[i].string != rhs.fragments_[i].string) {
       return false;
@@ -135,7 +135,7 @@ bool AttributedString::isContentEqual(const AttributedString &rhs) const {
     return false;
   }
 
-  for (auto i = 0; i < fragments_.size(); i++) {
+  for (size_t i = 0; i < fragments_.size(); i++) {
     if (!fragments_[i].isContentEqual(rhs.fragments_[i])) {
       return false;
     }