[lld] Avoid creating files outside of work directory.

[lld] Avoid creating files outside of work directory.

Summary:
A test is passing `-o -` to lld in the hope of writing the output to
standard out but that is not the case. Instead it creates a file named
`-.lto.o`. This fixes it by creating a temporary file in the work
directory.

Differential Revision: https://reviews.llvm.org/D75605