rustc: Get LLVM error message safely

On Windows, `LLVMRustGetLastError()` may return non-utf8 mojibake string
if system uses non-English locale. It caused ICE when llvm fails.
This patch doesn't fix the real problem, but just make rustc not die.
auto merge of #12732 : klutzy/rust/this-is-windows, r=alexcrichton

On Windows, `LLVMRustGetLastError()` may return non-utf8 mojibake string
if system uses non-English locale. It caused ICE when llvm fails.
This patch doesn't fix the real problem, but just make rustc not die.