[lld][unittests] Change assert to llvm_unreachable

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189023 91177308-0d34-0410-b5e6-96231b3b80d8
[lld][PECOFF] Fix use of temporary strings

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189025 91177308-0d34-0410-b5e6-96231b3b80d8