Use FromStr trait for number option parsing

Replace `parse_uint` with generic `parse_number` based on `FromStr`.
Use it for parsing inlining threshold to avoid casting later.

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -106,7 +106,7 @@ pub struct ModuleConfig {
     pub vectorize_loop: bool,
     pub vectorize_slp: bool,
     pub merge_functions: bool,
-    pub inline_threshold: Option<usize>,
+    pub inline_threshold: Option<u32>,
     pub new_llvm_pass_manager: bool,
     pub emit_lifetime_markers: bool,
 }