rustc_llvm: Fix flattened CLI args

Fixes string manipulation errors introduced in rust-lang#130446.

Author: aeubanks

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -491,12 +491,12 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
     auto Arg0 = std::string(ArgsCstrBuff);
     buffer_offset = Arg0.size() + 1;
     auto ArgsCppStr =
-        std::string(ArgsCstrBuff + buffer_offset, ArgsCstrBuffLen - 1);
+        std::string(ArgsCstrBuff + buffer_offset, ArgsCstrBuffLen - buffer_offset);
     auto i = 0;
     while (i != std::string::npos) {
       i = ArgsCppStr.find('\0', i + 1);
       if (i != std::string::npos)
-        ArgsCppStr.replace(i, i + 1, " ");
+        ArgsCppStr.replace(i, 1, " ");
     }
     Options.MCOptions.Argv0 = Arg0;
     Options.MCOptions.CommandlineArgs = ArgsCppStr;