[lldb][NFC] Make include directories in Clang expression parser a std::string

[lldb][NFC] Make include directories in Clang expression parser a std::string

We never compare these directories (where ConstString would be good) and
essentially just convert this back to a normal string in the end. So we might
as well just use std::string. Also makes it easier to unittest this code
(which was the main motivation for this change).

llvm-svn: 371623