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/tuple/assign.carbon

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

executable_semantics/testdata/tuple/ending_comma.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t1: (i32,) = (5,);
   var t2: (i32, i32) = (2, 3,);
   return t1[0] - t2[0] - t2[1];

executable_semantics/testdata/tuple/equality.carbon

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

executable_semantics/testdata/tuple/equality_false.carbon

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

executable_semantics/testdata/tuple/fail_equality_type.carbon

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

executable_semantics/testdata/tuple/fail_index.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: auto = (0, 1);
   return x[2];
 }

executable_semantics/testdata/tuple/fail_index_var.carbon

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

executable_semantics/testdata/tuple/index.carbon

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

executable_semantics/testdata/tuple/match.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: auto = (5, 2);
   match (t) {
     case (a: auto, b: auto) =>

executable_semantics/testdata/tuple/match_nested.carbon

@@ -13,7 +13,7 @@ package ExecutableSemanticsTest api;
 
 // Test matching of a tuple inside a tuple.
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var t: auto = ((1,2),(3,4));
   match (t) {
     case ((a: auto, b: auto), c: auto) =>

executable_semantics/testdata/tuple/no_ending_comma.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   var x: i32 = (1);
   var t2: (i32,i32) = (5, 2);
   t2[0] = 3;