[lit] Fix launching executables relative to the cwd after 'cd'

This was affecting test/asan/TestCases/Windows/coverage-basic.cc in
compiler-rt. It does something like:

  cd %T/mydir
  %clang %s -o t.exe
  ./t.exe

Previously, we'd end up looking for t.exe relative to the cwd of the lit
process, not the cwd of the test.

llvm-svn: 242941