If a hex literal has an 'd' of 'f' in it, such as '0xf' or '0xd', the compiler fails with illegal tree structure.

The other hex literals work.  The issue is probably related to using `f` and `d` in float and double literals.

```
    public void testHexCollisionDouble() {
        assertEquals(13, exec("return 0xd"));
    }

    public void testHexCollisionFloat() {
        assertEquals(15, exec("return 0xf"));
    }

    public void testHex() throws Exception {
        for (int i = 0; i <= 15; i++) {
            String hex = Integer.toHexString(i);
            if ("d".equals(hex) || "f".equals(hex)) {
                continue;
            }
            assertEquals(i, exec("return 0x" + hex));
            assertEquals(i, exec("return 0x" + hex.toUpperCase(Locale.ROOT)));
            assertEquals(i, exec("return 0X" + hex));
            assertEquals(i, exec("return 0X" + hex.toUpperCase(Locale.ROOT)));
        }
    }
```
modules/lang-painless/src/main/java/org/elasticsearch/painless/phase/DefaultSemanticAnalysisPhase.java
* Checks: type validation
     */
    @Override
    public void visitFunctionRef(EFunctionRef userFunctionRefNode, SemanticScope semanticScope) {
        ScriptScope scriptScope = semanticScope.getScriptScope();
        Location location = userFunctionRefNode.getLocation();
        String symbol = userFunctionRefNode.getSymbol();
        String methodName = userFunctionRefNode.getMethodName();
        boolean read = semanticScope.getCondition(userFunctionRefNode, Read.class);
        Class<?> type = scriptScope.getPainlessLookup().canonicalTypeNameToType(symbol);
        TargetType targetType = semanticScope.getDecoration(userFunctionRefNode, TargetType.class);
        Class<?> valueType;
        boolean isInstanceReference = "this".equals(symbol);
        if (isInstanceReference || type != null) {
            if (semanticScope.getCondition(userFunctionRefNode, Write.class)) {
                throw userFunctionRefNode.createError(
                    new IllegalArgumentException(
                        "invalid assignment: cannot assign a value to function reference [" + symbol + ":" + methodName + "]"
                    )
                );
            }
            if (read == false) {
                throw userFunctionRefNode.createError(
                    new IllegalArgumentException("not a statement: function reference [" + symbol + ":" + methodName + "] not used")
                );
            }
            if (isInstanceReference) {
                semanticScope.setCondition(userFunctionRefNode, InstanceCapturingFunctionRef.class);
            }
            if (targetType == null) {