[mycpp] Add parens around expressions.

Without this, we'll have operator precedence bugs.

test/spec-cpp: 856 passing under osh_eval.cc!  Up from 851, so it fixed
5 bugs.

mycpp/cppgen_pass.py

@@ -759,9 +759,16 @@ def visit_op_expr(self, o: 'mypy.nodes.OpExpr') -> T:
           self.write(')')
           return
 
+        # These parens are sometimes extra, but sometimes required.  Example:
+        #
+        # if ((a and (false or true))) {  # right
+        # vs.
+        # if (a and false or true)) {  # wrong
+        self.write('(')
         self.accept(o.left)
         self.write(' %s ', c_op)
         self.accept(o.right)
+        self.write(')')
 
     def visit_comparison_expr(self, o: 'mypy.nodes.ComparisonExpr') -> T:
         # Make sure it's binary

mycpp/examples/conditional.py

@@ -46,6 +46,14 @@ def run_tests():
   log("x = %d", x)
 
 
+  # Expressions where parens are needed
+  a = False
+  if a and (False or True):
+    print('yes')
+  else:
+    print('no')
+
+
 def run_benchmarks():
   # type: () -> None
   pass