Add support for simple object-like macros

[ivanaivanovska]

Adds support for object-like macros with a single replacement numeric-literal kind token. Only macros that evaluate to an integer constant are supported for now. When detected at name lookup, they are imported as a constant integer value in Carbon.

Demo:

// --- macros.h

#define CONFIG_VALUE 2

// main.carbon
library "Main";

import Cpp library "macros.h";
import Core library "io";

fn Run() {
    let a: i32 = Cpp.CONFIG_VALUE;
    Core.Print(a);
}

$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o \--output=demo_carbon
$ ./demo_carbon
2

Part of #6303

[zygoloid]

We should eventually allow an implicit conversion from i32 to f32, at which point I think this test won't test that the macro is being selected. How about something like:

Suggested change

	let b: f32 = Cpp.n;
	var b: array(f32, Cpp.n) = (1.0,);

... to test that it's being treated as an integer constant with value 1.

[ivanaivanovska]

Sg, thanks for the suggestion.

[zygoloid]

It'd be nice to do this identifier lookup only once, rather than once here and once in ClangLookupName. I'd be happy with a TODO if you'd like to get this landed to unblock further macro work.

[ivanaivanovska]

Added a TODO.