[MDC/LibraryInfo] Add "k" to string const to fix clang-tidy

Following http://clang.llvm.org/extra/clang-tidy/checks/google-objc-global-variable-declaration.html we should be doing this.

This also addresses the constant pointer issue that clang recommends.

PiperOrigin-RevId: 320390941

components/LibraryInfo/src/MDCLibraryInfo.m

@@ -19,12 +19,12 @@
 // This string is updated automatically as a part of the release process and should not be edited
 // manually. Do not rename this constant or change the formatting without updating the release
 // scripts.
-static NSString const *MDCLibraryInfoVersionString = @"110.2.0";
+static NSString* const kMDCLibraryInfoVersionString = @"110.2.0";
 
 @implementation MDCLibraryInfo
 
 + (NSString *)versionString {
-  return [MDCLibraryInfoVersionString copy];  // Copy because caller isn't expecting `const`.
+  return [kMDCLibraryInfoVersionString copy];  // Copy because caller isn't expecting `const`.
 }
 
 @end