[LTO] Add support for existing Config::Freestanding option.

lto::Config has a field to control whether the build is "freestanding"
(no builtins) or not, but it is not hooked up to the code actually
running the passes.

This patch adds support for the flag to both the code that runs
optimization with the new and old pass managers, by explicitly adding a
TargetLibraryInfo instance. If Freestanding is true, all library functions
are disabled.

Reviewed By: steven_wu

Differential Revision: https://reviews.llvm.org/D94630