Return pointer to non-const for getExecutionEngine

The `clang::Interpreter` wrapper previously returned a pointer to const,
but the actual `clang::Interpreter` did not. This breaks code for anyone
porting from `clang::Interpreter` to `Cpp::Interpreter`, especially
since most operations on the execution engine are mutations of some
sort.

Yes, the user could static cast the wrapper to the inner interpreter,
but, at that point, why both exposing the `getExecutionEngine` function
at all?

Author: jeaye

lib/Interpreter/CppInterOpInterpreter.h

@@ -167,7 +167,7 @@ class Interpreter {
     return inner->getCompilerInstance();
   }
 
-  const llvm::orc::LLJIT* getExecutionEngine() const {
+  llvm::orc::LLJIT* getExecutionEngine() const {
     return compat::getExecutionEngine(*inner);
   }