Rename main to Main for Carbon style consistency (#939)

Main() is more consistency with Carbon's naming guidelines. C# offers some precedent: https://docs.microsoft.com/en-us/dotnet/csharp/fundamentals/program-structure/main-command-line

executable_semantics/testdata/global_variable/write_from_function.carbon

@@ -20,7 +20,7 @@ fn flipFlag() {
   flag = 0;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   flipFlag();
   return flag;
 }