Don't generate wrong strings. (#2487)

A header with

```cpp
#include <stddef.h>

const char S1[] = "Hello";
const unsigned char S2[] = u8"Hello";
const char16_t S3[] = u"Hello";
const char32_t S4[] = U"Hello";
const wchar_t S5[] = L"Hello";
```

will wrongly generate (`-- -std=c++11`)


```rust
pub const S1: &[u8; 6usize] = b"Hello\0";
pub const S2: &[u8; 6usize] = b"Hello\0";
pub const S3: &[u8; 2usize] = b"H\0";
pub const S4: &[u8; 2usize] = b"H\0";
pub const S5: &[u8; 2usize] = b"H\0";
```

since `clang_EvalResult_getAsStr` only works for ordinary and UTF-8 strings. This seems like a `libclang` limitation since there isn't a similar function for other string literal types.

This disables generating code for unsupported string types.

CHANGELOG.md

@@ -186,12 +186,15 @@
  * The documentation of the generated `type` aliases now matches the comments
    of their `typedef` counterparts instead of using the comments of the aliased
    types. 
+
 ## Removed
  * The following deprecated flags were removed: `--use-msvc-mangling`,
    `--rustfmt-bindings` and `--size_t-is-usize`.
  * The `--no-rustfmt-bindings` flag was removed in favor of `--formatter=none`.
  * The `Bindings::emit_warnings` and `Bindings::warnings` methods were removed
    in favor of `--emit-diagnostics`.
+ * Bindgen no longer generates C string constants that cannot be represented as
+   byte slices.
 
 ## Fixed
 

bindgen/clang.rs

@@ -2104,6 +2104,7 @@ pub(crate) fn extract_clang_version() -> String {
 #[derive(Debug)]
 pub(crate) struct EvalResult {
     x: CXEvalResult,
+    ty: Type,
 }
 
 impl EvalResult {
@@ -2131,6 +2132,7 @@ impl EvalResult {
         }
         Some(EvalResult {
             x: unsafe { clang_Cursor_Evaluate(cursor.x) },
+            ty: cursor.cur_type().canonical_type(),
         })
     }
 
@@ -2177,13 +2179,22 @@ impl EvalResult {
     /// 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>> {
-        match self.kind() {
-            CXEval_StrLiteral => {
+        if self.kind() != CXEval_StrLiteral {
+            return None;
+        }
+
+        let char_ty = self.ty.pointee_type().or_else(|| self.ty.elem_type())?;
+        match char_ty.kind() {
+            CXType_Char_S | CXType_SChar | CXType_Char_U | CXType_UChar => {
                 let ret = unsafe {
                     CStr::from_ptr(clang_EvalResult_getAsStr(self.x))
                 };
                 Some(ret.to_bytes().to_vec())
             }
+            // FIXME: Support generating these.
+            CXType_Char16 => None,
+            CXType_Char32 => None,
+            CXType_WChar => None,
             _ => None,
         }
     }