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/class/assign.carbon

@@ -16,7 +16,7 @@ class Point {
   var y: i32;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var p1: Point = {.x = 1, .y = 2};
   var p2: auto = p1;
   p2 = {.x = 3, .y = 2};

executable_semantics/testdata/class/assign_member.carbon

@@ -16,7 +16,7 @@ class Point {
   var y: i32;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var p1: Point = {.x = 1, .y = 2};
   var p2: auto = p1;
   p2.x = 3;

executable_semantics/testdata/class/fail_field_access_mismatch.carbon

@@ -16,7 +16,7 @@ class Point {
   var y: i32;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var p: Point = {.x = 1, .y = 2};
   return p.z - 1;
 }

executable_semantics/testdata/class/fail_field_mismatch.carbon

@@ -16,7 +16,7 @@ class Point {
   var y: i32;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var p: Point = {.x = 1, .z = 2};
   return p.x - 1;
 }

executable_semantics/testdata/class/fail_field_missing.carbon

@@ -16,7 +16,7 @@ class Point {
   var y: i32;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var p: Point = {.x = 1};
   return p.x - 1;
 }

executable_semantics/testdata/class/function_param.carbon

@@ -20,6 +20,6 @@ fn GetX(p: Point) -> i32 {
   return p.x;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return GetX({.x = 1, .y = 2}) - 1;
 }

executable_semantics/testdata/class/global_var.carbon

@@ -18,6 +18,6 @@ class Point {
 
 var p: Point = {.x = 1, .y = 2};
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return p.y - p.x - 1;
 }

executable_semantics/testdata/class/temp.carbon

@@ -20,6 +20,6 @@ fn MakePoint() -> Point {
   return {.x = 1, .y = 2};
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return MakePoint().x - 1;
 }

executable_semantics/testdata/class/var.carbon

@@ -16,7 +16,7 @@ class Point {
   var y: i32;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var p: Point = {.x = 1, .y = 2};
   return p.y - p.x - 1;
 }