Uses `clang::Parser::ParseConstantExpression()` to parse the macro
replacement tokens, added as a token stream to the preprocessor. This
extends the support from simple object-like macros with a single
replacement token, to multiple tokens like unary operators, binary
operators, casting, nested macros etc.
The support is still limited to macros that are evaluated to an integer
constant. More types to be added as a follow-up.
Part of #6303
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:
```c++
// --- macros.h
#define CONFIG_VALUE 2
```
``` c++
// main.carbon
library "Main";
import Cpp library "macros.h";
import Core library "io";
fn Run() {
let a: i32 = Cpp.CONFIG_VALUE;
Core.Print(a);
}
```
```c++
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o \--output=demo_carbon
$ ./demo_carbon
2
```
Part of #6303
Ivana Ivanovska