[lldb] Make sure import-std-module/sysroot actually passes for the right reasons

This test was previously passing because myabs was actually emitted into the
debug information and we called that. The test itself was broken as it didn't
use the libc++ directory structure (the /v1/ directory was just called /include/).

This patch gives myabs a default argument which we can't get from debug information
and inlines the function to make sure we can't call it from LLDB without loading
the C++ module.

llvm-svn: 374335