[LTO] Handle Task=-1 passed to addSaveTemps

Summary:
This change is necessary for D46464, which will pass -1 as the Task
ID for distributed backends, so that the save temps files don't end
up with "4294967295" in their path. For distributed back ends, when -1
is passed, don't append any Task ID.

An existing test (tools/clang/test/CodeGen/thinlto_backend.ll) will
fail without this change after D46464.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D46488

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331591 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed