[lldb] Add nodebug attribute to import-std-module/sysroot test

Summary:
So far we rely on the default argument and the fact that we don't call this
inline function in our actual `main.cpp` to make sure that this function can only
be called if LLDB loads this header as a C++ module. This patch just adds
the nodebug attribute as yet another measure to make sure LLDB can't call this
function without the standard module loaded. Note that the test is already
requiring clang for the sysroot setup, so its fine that this is a Clang specific attribute.

Reviewers: friss, labath

Subscribers: JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68861