Change SIL box mangling prefix to Xb.

Now that we start with 'X', we can use 'b' for box.

Suggested by @jrose-apple.

Author: rudkx

lib/AST/Mangle.cpp

@@ -881,7 +881,7 @@ void Mangler::mangleAssociatedTypeName(DependentMemberType *dmt,
 /// <type> ::= Xo <type>             # unowned reference type
 /// <type> ::= Xw <type>             # weak reference type
 /// <type> ::= XF <impl-function-type> # SIL function type
-/// <type> ::= XH <type>             # SIL @box type
+/// <type> ::= Xb <type>             # SIL @box type
 ///
 /// <index> ::= _                    # 0
 /// <index> ::= <natural> _          # N+1
@@ -1395,7 +1395,7 @@ void Mangler::mangleType(Type type, ResilienceExpansion explosion,
   }
 
   case TypeKind::SILBox:
-    Buffer << 'X' << 'H';
+    Buffer << 'X' << 'b';
     mangleType(cast<SILBoxType>(tybase)->getBoxedType(), explosion,
                uncurryLevel);
     return;

lib/Basic/Demangle.cpp

@@ -1884,7 +1884,7 @@ class Demangler {
       return type_application;
     }
     if (c == 'X') {
-      if (Mangled.nextIf('H')) {
+      if (Mangled.nextIf('b')) {
         NodePointer type = demangleType();
         if (!type)
           return nullptr;

lib/Basic/Remangle.cpp

@@ -1161,7 +1161,7 @@ void Remangler::mangleErrorType(Node *node) {
 }
 
 void Remangler::mangleSILBoxType(Node *node) {
-  Out << 'X' << 'H';
+  Out << 'X' << 'b';
   mangleSingleChildNode(node);
 }
 

test/SILOptimizer/closure_specialize.sil

@@ -226,7 +226,7 @@ bb3:
 // Ensure that we can specialize and properly mangle functions that take closures with @box arguments.
 
 
-// CHECK-LABEL: sil shared [noinline] @_TTSf1n_cl25closure_with_box_argumentXHBi32____TF4main5innerFTRVs5Int32FS0_T__T_ : $@convention(thin) (@inout Builtin.Int32, @owned @box Builtin.Int32) -> ()
+// CHECK-LABEL: sil shared [noinline] @_TTSf1n_cl25closure_with_box_argumentXbBi32____TF4main5innerFTRVs5Int32FS0_T__T_ : $@convention(thin) (@inout Builtin.Int32, @owned @box Builtin.Int32) -> ()
 // CHECK: bb0
 // CHECK: [[FN:%.*]] = function_ref @closure_with_box_argument
 // CHECK: [[PARTIAL:%.*]] = partial_apply [[FN]](%1)