Sema: factor out NeededComptimeReason from comptime value resolution

This makes the call sites easier to read, reduces the number of `catch`
expressions required, and prepares for comptime reasons to appear
earlier in the list of notes.

src/Module.zig

@@ -4147,7 +4147,9 @@ fn semaDecl(mod: *Module, decl_index: Decl.Index) !bool {
     const address_space_src: LazySrcLoc = .{ .node_offset_var_decl_addrspace = 0 };
     const ty_src: LazySrcLoc = .{ .node_offset_var_decl_ty = 0 };
     const init_src: LazySrcLoc = .{ .node_offset_var_decl_init = 0 };
-    const decl_tv = try sema.resolveInstValue(&block_scope, init_src, result_ref, "global variable initializer must be comptime-known");
+    const decl_tv = try sema.resolveInstValue(&block_scope, init_src, result_ref, .{
+        .needed_comptime_reason = "global variable initializer must be comptime-known",
+    });
 
     // Note this resolves the type of the Decl, not the value; if this Decl
     // is a struct, for example, this resolves `type` (which needs no resolution),
@@ -4257,7 +4259,9 @@ fn semaDecl(mod: *Module, decl_index: Decl.Index) !bool {
     decl.@"linksection" = blk: {
         const linksection_ref = decl.zirLinksectionRef(mod);
         if (linksection_ref == .none) break :blk .none;
-        const bytes = try sema.resolveConstString(&block_scope, section_src, linksection_ref, "linksection must be comptime-known");
+        const bytes = try sema.resolveConstString(&block_scope, section_src, linksection_ref, .{
+            .needed_comptime_reason = "linksection must be comptime-known",
+        });
         if (mem.indexOfScalar(u8, bytes, 0) != null) {
             return sema.fail(&block_scope, section_src, "linksection cannot contain null bytes", .{});
         } else if (bytes.len == 0) {