Only use major version in the clang resource directory for standalone builds.

Commit e1b88c8a changed the name of the clang resource directory so that
it was "lib/clang/<MajorVersion>" but missed the place in the LLDB standalone
build where we search for the resource directory.  That was still looking for
<Major>.<Minor>.<Patch>.  The standalone lldb bot has been failing since this
commit.