Convert brand-pointer to constexpr-function (1/3)

MSVC erroneously refuses to compile code which calculates the address of
objects of static storage duration at compile-time, despite this being
legal. Work around this by changing `_capnpPrivate::brand` from constexpr
pointers to static constexpr functions returning pointers. The compiler
is then free to calculate it at compile-time if it can (gcc, clang), but
can also defer it to run-time if it can't (MSVC).

This change is split into three commits:

1. Changes to the code generator (this commit).
2. A bootstrap regeneration, after which the codebase will not compile.
3. Changes to the generated code support header, after which the codebase
will again compile.

c++/src/capnp/compiler/capnpc-c++.c++

@@ -792,7 +792,7 @@ private:
     if (type.isBranded()) {
       name.addMemberType("_capnpPrivate");
       name.addMemberValue("brand");
-      return kj::strTree(name);
+      return kj::strTree(name, "()");
     } else {
       return nullptr;
     }
@@ -1800,7 +1800,7 @@ private:
         "\n"
         "#if !CAPNP_LITE\n"
         "  inline ::kj::StringTree toString() const {\n"
-        "    return ::capnp::_::structString(_reader, *_capnpPrivate::brand);\n"
+        "    return ::capnp::_::structString(_reader, *_capnpPrivate::brand());\n"
         "  }\n"
         "#endif  // !CAPNP_LITE\n"
         "\n",
@@ -1891,8 +1891,8 @@ private:
         (!hasBrandDependencies ? "" :
             "    static const ::capnp::_::RawBrandedSchema::Dependency brandDependencies[];\n"),
         "    static const ::capnp::_::RawBrandedSchema specificBrand;\n"
-        "    static constexpr ::capnp::_::RawBrandedSchema const* brand = "
-        "::capnp::_::ChooseBrand<_capnpPrivate, ", templateContext.allArgs(), ">::brand;\n");
+        "    static constexpr ::capnp::_::RawBrandedSchema const* brand() { "
+        "return ::capnp::_::ChooseBrand<_capnpPrivate, ", templateContext.allArgs(), ">::brand(); }\n");
   }
 
   kj::StringTree makeGenericDefinitions(
@@ -1953,8 +1953,7 @@ private:
         templates, "constexpr uint16_t ", fullName, "::_capnpPrivate::pointerCount;\n"
         "#if !CAPNP_LITE\n",
         templates, "constexpr ::capnp::Kind ", fullName, "::_capnpPrivate::kind;\n",
-        templates, "constexpr ::capnp::_::RawSchema const* ", fullName, "::_capnpPrivate::schema;\n",
-        templates, "constexpr ::capnp::_::RawBrandedSchema const* ", fullName, "::_capnpPrivate::brand;\n");
+        templates, "constexpr ::capnp::_::RawSchema const* ", fullName, "::_capnpPrivate::schema;\n");
 
     if (templateContext.isGeneric()) {
       auto brandInitializers = makeBrandInitializers(templateContext, schema);
@@ -1970,7 +1969,7 @@ private:
     } else {
       declareText = kj::strTree(kj::mv(declareText),
           "    #if !CAPNP_LITE\n"
-          "    static constexpr ::capnp::_::RawBrandedSchema const* brand = &schema->defaultBrand;\n"
+          "    static constexpr ::capnp::_::RawBrandedSchema const* brand() { return &schema->defaultBrand; }\n"
           "    #endif  // !CAPNP_LITE\n");
     }
 
@@ -2228,8 +2227,7 @@ private:
         "// ", fullName, "\n",
         "#if !CAPNP_LITE\n",
         templates, "constexpr ::capnp::Kind ", fullName, "::_capnpPrivate::kind;\n",
-        templates, "constexpr ::capnp::_::RawSchema const* ", fullName, "::_capnpPrivate::schema;\n",
-        templates, "constexpr ::capnp::_::RawBrandedSchema const* ", fullName, "::_capnpPrivate::brand;\n");
+        templates, "constexpr ::capnp::_::RawSchema const* ", fullName, "::_capnpPrivate::schema;\n");
 
     if (templateContext.isGeneric()) {
       auto brandInitializers = makeBrandInitializers(templateContext, schema);
@@ -2242,7 +2240,7 @@ private:
           makeGenericDefinitions(templates, fullName, kj::str(hexId), kj::mv(brandInitializers)));
     } else {
       declareText = kj::strTree(kj::mv(declareText),
-        "    static constexpr ::capnp::_::RawBrandedSchema const* brand = &schema->defaultBrand;\n");
+        "    static constexpr ::capnp::_::RawBrandedSchema const* brand() { return &schema->defaultBrand; }\n");
     }
 
     declareText = kj::strTree(kj::mv(declareText), "  };\n  #endif  // !CAPNP_LITE");
@@ -2644,7 +2642,8 @@ private:
             kj::StringTree(KJ_MAP(index, membersByDiscrim) { return kj::strTree(index); }, ", "),
             "};\n"),
         brandDeps.size() == 0 ? kj::strTree() : kj::strTree(
-            "const ::capnp::_::RawBrandedSchema::Dependency bd_", hexId, "[] = ", kj::mv(brandDeps), ";\n"),
+            "KJ_CONSTEXPR(const) ::capnp::_::RawBrandedSchema::Dependency bd_", hexId, "[] = ",
+            kj::mv(brandDeps), ";\n"),
         "const ::capnp::_::RawSchema s_", hexId, " = {\n"
         "  0x", hexId, ", b_", hexId, ".words, ", rawSchema.size(), ", ",
         deps.size() == 0 ? kj::strTree("nullptr") : kj::strTree("d_", hexId), ", ",