[libLTO] Set data-sections by default in libLTO.

[libLTO] Set data-sections by default in libLTO.

This patch changes legacy LTO to set data-sections by default. The user can
explicitly unset data-sections. The reason for this patch is to match the
behaviour of lld and gold plugin. Both lld and gold plugin have data-sections on
by default.

This patch also fixes the forwarding of the clang options -fno-data-sections and
-fno-function-sections to libLTO. Now, when -fno-data/function-sections are
specified in clang, -data/function-sections=0 will be passed to libLTO to
explicitly unset data/function-sections.

Reviewed By: w2yehia, MaskRay

Differential Revision: https://reviews.llvm.org/D129401