Add testing for C decl source range extraction

[fw-immunant]

Includes some other minor cleanups made on the way.

[kkysen]

Can we just enable this for everything in fn config? And then refactor the non-snapshot part of fn transpile/fn transpile_snapshot into a shared function, because they're basically both the same except for the end that does the snapshot testing.

[fw-immunant]

I've factored out the shared code, but setting emit_c_decls_json for all snapshots generates a bunch of irrelevant output files, so I think it's better to only set it when we actually want that output.

[kkysen]

What's wrong with some extra .gitignored output files? This adds some more testing at what is IMO negligible cost (some extra ignored files).

[kkysen]

Suggested change

	// Safety: Not thread-safe; must not be called multiple times concurrently.
	/// # Safety
	///
	/// Not thread-safe; must not be called multiple times concurrently.

[kkysen]

Suggested change

	anotherfunc(void)
	{
	int var = 0;
	return 4 + var;
	}
	int
	funcholdingdefine(void)
	{
	#define FOO 4000+50
	return FOO;
	}
	// comment before multiple decl
	int a1, a2, a3;
	void forwarddeclbeforeotherdef(int x);
	void funcafterdecl(int n) {}
	/*real defn*/
	void forwarddeclbeforeotherdef(int x) { }
	another_func(void)
	{
	int var = 0;
	return 4 + var;
	}
	int
	func_holding_define(void)
	{
	#define FOO 4000+50
	return FOO;
	}
	// comment before multiple decl
	int a1, a2, a3;
	void forward_decl_before_other_def(int x);
	void func_after_decl(int n) {}
	/*real defn*/
	void forward_decl_before_other_def(int x) { }

Was there a reason these names didn't have separators? It's pretty hard to read without them.