Clean up remaining uses of old spelling main (#959)

executable_semantics/testdata/name_lookup/fail_block_duplicate.carbon

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

executable_semantics/testdata/name_lookup/fail_choice_duplicate.carbon

@@ -16,6 +16,6 @@ choice Ints {
   None,
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return 0;
 }

executable_semantics/testdata/name_lookup/fail_class_duplicate.carbon

@@ -16,6 +16,6 @@ class Foo {
   var x: i32;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return 0;
 }

executable_semantics/testdata/name_lookup/fail_global_duplicate.carbon

@@ -14,6 +14,6 @@ package ExecutableSemanticsTest api;
 var x: i32 = 0;
 var x: i32 = 0;
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return 0;
 }

executable_semantics/testdata/name_lookup/fail_match_duplicate.carbon

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