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/struct/assign.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: auto = {.x = 0, .y = 1};
   x = {.x = 5, .y = -5};
   return x.x + x.y;

executable_semantics/testdata/struct/assign_member.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var p1: auto = {.x = 1, .y = 2};
   var p2: auto = p1;
   p2.x = 3;

executable_semantics/testdata/struct/empty.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var empty: {} = {};
   empty = {};
   if (not (empty == {})) {

executable_semantics/testdata/struct/ending_comma.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t1: {.x: i32,} = {.x = 5,};
   var t2: {.x: i32, .y: i32} = {.x = 2, .y = 3,};
   return t1.x - t2.x - t2.y;

executable_semantics/testdata/struct/equality.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t1: {.x: i32, .y: i32} = {.x = 5, .y = 2};
   var t2: {.x: i32, .y: i32} = {.x = 5, .y = 2};
   if (t1 == t2) {

executable_semantics/testdata/struct/equality_false.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t1: {.x: i32, .y: i32} = {.x = 5, .y = 2};
   var t2: {.x: i32, .y: i32} = {.x = 5, .y = 4};
   if (t1 == t2) {

executable_semantics/testdata/struct/fail_equality_type.carbon

@@ -10,7 +10,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t1: {.x: i32, .y: i32} = {.x = 5, .y = 2};
   var t2: {.x: i32,} = {.x = 5,};
   if (t1 == t2) {

executable_semantics/testdata/struct/fail_field_access_mismatch.carbon

@@ -11,6 +11,6 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return {.x = 1, .y = 2}.z - 1;
 }

executable_semantics/testdata/struct/name_order.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test the that field order doesn't matter for structs.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: {.x: i32, .y: i32} = {.y = 2, .x = 3};
   return 0;
 }

executable_semantics/testdata/struct/temp.carbon

@@ -11,6 +11,6 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return {.x = 1, .y = 2}.x - 1;
 }

executable_semantics/testdata/struct/var.carbon

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