add error message for -fno-llvm -flld

Zig will never support linking in stage2 with LLD as the self-hosted backends are linker driven.
We plan to remove all dependency on LLD either way, so this will not be a supported usecase.

src/Compilation/Config.zig

@@ -119,6 +119,7 @@ pub const ResolveError = error{
  ZigLacksTargetSupport,
  EmittingBinaryRequiresLlvmLibrary,
  LldIncompatibleObjectFormat,
+ LldIncompatibleWithStage2,
  LtoRequiresLld,
  SanitizeThreadRequiresLibCpp,
  LibCppRequiresLibUnwind,
@@ -257,6 +258,11 @@ pub fn resolve(options: Options) ResolveError!Config {
  break :b true;
  }
 
+ if (options.use_llvm == false) {
+ if (options.use_lld == true) return error.LldIncompatibleWithStage2;
+ break :b false;
+ }
+
  if (options.use_lld) |x| break :b x;
  break :b true;
  };

src/main.zig

@@ -3931,6 +3931,7 @@ fn createModule(
  error.ZigLacksTargetSupport => fatal("compiler backend unavailable for the specified target", .{}),
  error.EmittingBinaryRequiresLlvmLibrary => fatal("producing machine code via LLVM requires using the LLVM library", .{}),
  error.LldIncompatibleObjectFormat => fatal("using LLD to link {s} files is unsupported", .{@tagName(target.ofmt)}),
+ error.LldIncompatibleWithStage2 => fatal("self-hosted backends do not support linking with LLD", .{}),
  error.LtoRequiresLld => fatal("LTO requires using LLD", .{}),
  error.SanitizeThreadRequiresLibCpp => fatal("thread sanitization is (for now) implemented in C++, so it requires linking libc++", .{}),
  error.LibCppRequiresLibUnwind => fatal("libc++ requires linking libunwind", .{}),