Make -Z new-llvm-pass-manager an Option<bool>

To allow it to have an LLVM version dependent default.

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -108,7 +108,7 @@ pub struct ModuleConfig {
     pub vectorize_slp: bool,
     pub merge_functions: bool,
     pub inline_threshold: Option<u32>,
-    pub new_llvm_pass_manager: bool,
+    pub new_llvm_pass_manager: Option<bool>,
     pub emit_lifetime_markers: bool,
 }