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/generic_function/apply.carbon

@@ -23,6 +23,6 @@ fn positive(x: Bool) -> i32 {
   }
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return apply(positive, false);
 }

executable_semantics/testdata/generic_function/fail_not_addable.carbon

@@ -17,6 +17,6 @@ fn id[T:! Type](x: T) -> T {
   return x + 0;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return id(0);
 }

executable_semantics/testdata/generic_function/fail_type_deduction_mismatch.carbon

@@ -17,6 +17,6 @@ fn fst[T:! Type](x: T, y: T) -> T {
   return x;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return fst(0, true);
 }

executable_semantics/testdata/generic_function/fail_type_deduction_unused.carbon

@@ -15,6 +15,6 @@ fn id[T:! Type](x: i32) -> i32 {
   return x;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return id(0);
 }

executable_semantics/testdata/generic_function/non_generic_param.carbon

@@ -15,6 +15,6 @@ fn snd[T:! Type](x: i32, y: T) -> T {
   return y;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return snd(0, 1);
 }

executable_semantics/testdata/generic_function/return_val.carbon

@@ -15,6 +15,6 @@ fn id[T:! Type](x: T) -> T {
   return x;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return id(0);
 }

executable_semantics/testdata/generic_function/swap.carbon

@@ -15,6 +15,6 @@ fn swap[T:! Type, U:! Type](tuple: (T, U)) -> (U, T) {
   return (tuple[1], tuple[0]);
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return swap((0, true))[1];
 }

executable_semantics/testdata/generic_function/tuple_map.carbon

@@ -17,6 +17,6 @@ fn map[T:! Type](f: __Fn (T) -> T, tuple: (T, T)) -> (T, T) {
 
 fn inc(x: i32) -> i32 { return x + 1; }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return map(inc, (0, 2))[0];
 }

executable_semantics/testdata/generic_function/type_matching.carbon

@@ -15,6 +15,6 @@ fn fst[T:! Type](x: T, y: T) -> T {
   return x;
 }
 
-fn main() -> i32 {
+fn Main() -> i32 {
   return fst(0, 1);
 }