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/match/any_int.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: auto = 5;
   match (t) {
     case x: i32 =>

executable_semantics/testdata/match/int.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: auto = 5;
   match (t) {
     case 5 =>

executable_semantics/testdata/match/int_default.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: auto = 5;
   match (t) {
     case 3 =>

executable_semantics/testdata/match/no_match.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: i32 = 0;
   match (x) {
     case 1 =>

executable_semantics/testdata/match/placeholder.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: auto = (1, 2, 3, 4);
   match (t) {
     case (_: i32, _: auto, x: i32, y: auto) =>