Allow macro definitions to pick up precise C types

Upon activating the `--macro-const-use-ctypes` flag, `bindgen` should
generate bindings with precise type resolution to the exact C types.

This flag implies `--clang-macro-fallback` as its pre-requisite.

Signed-off-by: Xiangfei Ding <dingxiangfei2009@protonmail.ch>

Author: dingxiangfei2009

bindgen-tests/tests/expectations/tests/issue-753.rs

@@ -7,6 +7,6 @@
 
 #define CONST UINT32_C(5)
 #define OTHER_CONST UINT32_C(6)
-#define LARGE_CONST UINT32_C(6 << 8)
+#define LARGE_CONST (UINT32_C(6) << 8)
 
 #endif

bindgen-tests/tests/expectations/tests/issue-923.rs

@@ -0,0 +1,4 @@
+// bindgen-flags: --macro-const-use-ctypes
+#define ULONG_ZERO 0UL
+#define ULONGLONG_ZERO 0ULL
+#define CHAR_ZERO ((char)'\0')

bindgen-tests/tests/expectations/tests/test_macro_fallback_non_system_dir.rs

@@ -2308,8 +2308,8 @@ impl EvalResult {
         {
             let mut found_cant_eval = false;
             cursor.visit(|c| {
-                if c.kind() == CXCursor_TypeRef &&
-                    c.cur_type().canonical_type().kind() == CXType_Unexposed
+                if c.kind() == CXCursor_TypeRef
+                    && c.cur_type().canonical_type().kind() == CXType_Unexposed
                 {
                     found_cant_eval = true;
                     return CXChildVisit_Break;
@@ -2328,7 +2328,7 @@ impl EvalResult {
         })
     }
 
-    fn kind(&self) -> CXEvalResultKind {
+    pub(crate) fn kind(&self) -> CXEvalResultKind {
         unsafe { clang_EvalResult_getKind(self.x) }
     }
 
@@ -2369,6 +2369,26 @@ impl EvalResult {
         Some(value as i64)
     }
 
+    pub(crate) fn as_str_literal(&self) -> Option<Vec<u8>> {
+        if !matches!(
+            self.kind(),
+            CXEval_StrLiteral | CXEval_CFStr | CXEval_ObjCStrLiteral,
+        ) {
+            return None;
+        }
+        // Safety: we are only copying the content, not assuming a borrow.
+        // TODO(@dingxiangfei2009): LLVM Libclang does not return the true size
+        // of a string literal, which could be truncated due to a null character
+        // '\0' in the middle.
+        let value =
+            unsafe { CStr::from_ptr(clang_EvalResult_getAsStr(self.x)) };
+        Some(value.to_bytes().into())
+    }
+
+    pub(crate) fn value_type(&self) -> Type {
+        self.ty
+    }
+
     /// Evaluates the expression as a literal string, that may or may not be
     /// valid utf-8.
     pub(crate) fn as_literal_string(&self) -> Option<Vec<u8>> {