add addCheckFile method to FileSource

[andrewrk]

Extracted from #14647.

zig/lib/std/Build.zig

Lines 1532 to 1539 in b4d58e9

	/// A file source is a reference to an existing or future file.
	pub const FileSource = union(enum) {
	/// A plain file path, relative to build root or absolute.
	path: []const u8,
	/// A file that is generated by an interface. Those files usually are
	/// not available until built by a build step.
	generated: *const GeneratedFile,

This new function should create a CheckFileStep.

Related: #14945

[bsubei]

Just a note for future readers, FileSource was renamed to LazyPath (see #16353).

[cdeler]

@andrewrk , I see you added

zig/lib/std/Build.zig

Lines 1859 to 1865 in 136c5a9

	pub fn addCheckFile(
	b: *Build,
	file_source: LazyPath,
	options: Step.CheckFile.Options,
	) *Step.CheckFile {
	return Step.CheckFile.create(b, file_source, options);
	}

since the issue was created (and this function is actively used). So probably it's not required anymore?