frontend: move BuiltinFn to std.zig namespace

lib/std/zig.zig

@@ -17,6 +17,7 @@ pub const primitives = @import("zig/primitives.zig");
 pub const Ast = @import("zig/Ast.zig");
 pub const system = @import("zig/system.zig");
 pub const CrossTarget = @import("zig/CrossTarget.zig");
+pub const BuiltinFn = @import("zig/BuiltinFn.zig");
 
 // Character literal parsing
 pub const ParsedCharLiteral = string_literal.ParsedCharLiteral;

src/AstGen.zig

@@ -13,7 +13,7 @@ const StringIndexContext = std.hash_map.StringIndexContext;
 const isPrimitive = std.zig.primitives.isPrimitive;
 
 const Zir = @import("Zir.zig");
-const BuiltinFn = @import("BuiltinFn.zig");
+const BuiltinFn = std.zig.BuiltinFn;
 const AstRlAnnotate = @import("AstRlAnnotate.zig");
 
 gpa: Allocator,

src/AstRlAnnotate.zig

@@ -18,7 +18,7 @@ const AstRlAnnotate = @This();
 const Ast = std.zig.Ast;
 const Allocator = std.mem.Allocator;
 const AutoHashMapUnmanaged = std.AutoHashMapUnmanaged;
-const BuiltinFn = @import("BuiltinFn.zig");
+const BuiltinFn = std.zig.BuiltinFn;
 const assert = std.debug.assert;
 
 gpa: Allocator,

src/Module.zig

@@ -34,7 +34,7 @@ const isUpDir = @import("introspect.zig").isUpDir;
 const clang = @import("clang.zig");
 const InternPool = @import("InternPool.zig");
 const Alignment = InternPool.Alignment;
-const BuiltinFn = @import("BuiltinFn.zig");
+const BuiltinFn = std.zig.BuiltinFn;
 
 comptime {
     @setEvalBranchQuota(4000);

src/reduce/Walk.zig

@@ -2,7 +2,7 @@ const std = @import("std");
 const Ast = std.zig.Ast;
 const Walk = @This();
 const assert = std.debug.assert;
-const BuiltinFn = @import("../BuiltinFn.zig");
+const BuiltinFn = std.zig.BuiltinFn;
 
 ast: *const Ast,
 transformations: *std.ArrayList(Transformation),