[RISCV][DebugInfo] Fix dwarf-riscv-relocs.ll test on Windows

Windows sees DW_AT_decl_file (".\dwarf-riscv-relocs.c") while Linux sees
DW_AT_decl_file ("./dwarf-riscv-relocs.c").

This fixes a failure introduced in rL366402.

llvm-svn: 366410