fix bug in parsing of non-ascii constants

c++/src/capnp/test.capnp

@@ -748,6 +748,8 @@ const embeddedData :Data = embed "testdata/packed";
 const embeddedText :Text = embed "testdata/short.txt";
 const embeddedStruct :TestAllTypes = embed "testdata/binary";
 
+const nonAsciiText :Text = "♫ é ✓";
+
 struct TestAnyPointerConstants {
   anyKindAsStruct @0 :AnyPointer;
   anyStructAsStruct @1 :AnyStruct;

c++/src/kj/parse/char.h

@@ -107,7 +107,7 @@ public:
     return CharGroup_(~bits[0], ~bits[1], ~bits[2], ~bits[3]);
   }
 
-  constexpr inline bool contains(char c) const {
+  constexpr inline bool contains(unsigned char c) const {
     return (bits[c / 64] & (1ll << (c % 64))) != 0;
   }