[lldb][clang] Reflect LangStandard.h move to clang/Basic

D65562 <https://reviews.llvm.org/D65562> moves LangStandard.h from clang/Frontend to clang/Basic.  This patch
adjusts the single file in lldb that uses it to match.

Tested on x86_64-pc-linux-gnu.

Differential Revision: https://reviews.llvm.org/D65717

llvm-svn: 367865