Support building RexLiterals from Character values

@Test void testCharacterLiteral() {
  char c = 'c';
  relBuilder.literal(c);
} 

  cannot convert c (class java.lang.Character) to a constant
java.lang.IllegalArgumentException: cannot convert c (class java.lang.Character) to a constant
    at org.apache.calcite.tools.RelBuilder.literal(RelBuilder.java:498)
    at org.apache.calcite.plan.RelOptUtilTest.testCharacterLiteral(RelOptUtilTest.java:410)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:727)
    at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
    at org.junit.jupiter.engine.extension.SameThreadTimeoutInvocation.proceed(SameThreadTimeoutInvocation.java:45)
    at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:156)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:147)