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/function/ignored_parameter.carbon

@@ -15,6 +15,6 @@ fn ReturnSecond(_: i32, x: i32) -> i32 {
   return x;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return ReturnSecond(1, 0);
 }