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/basic_syntax/choice.carbon

@@ -17,7 +17,7 @@ choice Ints {
   Two(i32,i32)
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: auto = Ints.None();
   var y: auto = Ints.One(42);
   var n: auto = 0;

executable_semantics/testdata/basic_syntax/fail_missing_var.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   // error
   x : i32;
   return 1;

executable_semantics/testdata/basic_syntax/next.carbon

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

executable_semantics/testdata/basic_syntax/placeholder_variable.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var _: auto = 1;
   return 0;
 }

executable_semantics/testdata/basic_syntax/print.carbon

@@ -12,7 +12,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var s: auto = "Hello world!\n";
   Print(s);
   return 0;

executable_semantics/testdata/basic_syntax/record.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t2: {.x: i32, .y: i32} = {.x = 2, .y = 5};
   t2.y = 3;
   return t2.y - t2.x - 1; // 3 - 2 - 1

executable_semantics/testdata/basic_syntax/star.carbon

@@ -20,6 +20,6 @@ fn F(n: i32, p: i32*, q: i32***) -> i32* {
   return **q;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return 0;
 }

executable_semantics/testdata/basic_syntax/var_tuple.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var (x: auto, y: auto) = (2, 3);
   return y - x - 1;
 }

executable_semantics/testdata/basic_syntax/zero.carbon

@@ -11,6 +11,6 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return 0;
 }