Auto merge of #38027 - rkruppe:llvm-printpasses-fwdcompat, r=alexcrichton
Auto merge of #38027 - rkruppe:llvm-printpasses-fwdcompat, r=alexcrichton

[LLVM 4.0] LLVMRustPrintPasses

Adapt `LLVMRustPrintPasses` to LLVM 4.0 preferring `StringRef` over `char *`

cc #37609

aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust