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/global_variable/fail_init_order.carbon

@@ -18,6 +18,6 @@ var x: i32 = y;
 
 var y: i32 = 0;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return x;
 }

executable_semantics/testdata/global_variable/fail_init_type_mismatch.carbon

@@ -15,6 +15,6 @@ package ExecutableSemanticsTest api;
 
 var flag: i32 = true;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return 0;
 }

executable_semantics/testdata/global_variable/init_and_read.carbon

@@ -15,6 +15,6 @@ package ExecutableSemanticsTest api;
 
 var zero: i32 = 0;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return zero;
 }

executable_semantics/testdata/global_variable/init_from_function.carbon

@@ -19,6 +19,6 @@ fn f() -> i32 {
 
 var y: i32 = f();
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return y;
 }

executable_semantics/testdata/global_variable/init_order.carbon

@@ -17,6 +17,6 @@ var x: i32 = 0;
 
 var y: i32 = x;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return y;
 }

executable_semantics/testdata/global_variable/shadowing.carbon

@@ -19,6 +19,6 @@ fn identity(x: i32) -> i32 {
   return x;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return identity(0);
 }

executable_semantics/testdata/global_variable/write.carbon

@@ -15,7 +15,7 @@ package ExecutableSemanticsTest api;
 
 var zero: i32 = 1;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   zero = 0;
   return zero;
 }

executable_semantics/testdata/global_variable/write_from_function.carbon

@@ -20,7 +20,7 @@ fn flipFlag() {
   flag = 0;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   flipFlag();
   return flag;
 }