Failed to copy wide char string literal

Example C++ code:
```c++
void sub(const wchar_t *a)
{
}

void top()
{
sub(L"a");
}
```
The prefix `L` in `L"a"`specifies `"a"` is wide char string. When using `SageInterface::copyExpression` to copy such string literals, the prefix `L` is dropped.