[yaml] Add a ScalarTraits for mapping endian aware types.

[zturner]

Added some unit tests.

[majnemer]

I'm a little confused. It seems to me that this wraps the endian-aware types so perfectly that there is not an effective difference between ulittle32_t and uint32_t.

[zturner]

The difference is that without this patch, the following does not compile:

ulittle32_t X;
IO.mapRequired("u32", X);

Sure, I could map it to a uint32_t, but all my internal data structures are
represented with iendian aware types, so this allows me to interact with
existing code more cleanly

[majnemer]

OK. So the actual use case is not:

ulittle32_t X;

but:

type_embedded_with_ulittle32 X;

?

If so, LGTM

[zturner]

Essentially. One concrete example is vector<ulittle32_t>, without this
patch it's very onerous to create one of these

include/llvm/Support/YAMLTraits.h

 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/Endian.h"
 #include "llvm/Support/Regex.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/YAMLParser.h"

   static bool mustQuote(StringRef) { return false; }
 };
 
+// For endian types, we just use the existing ScalarTraits for the underlying
+// type.  This way endian aware types are supported whenever a ScalarTraits
+// is defined for the underlying type.
+template <typename value_type, support::endianness endian, size_t alignment>
+struct ScalarTraits<support::detail::packed_endian_specific_integral<
+    value_type, endian, alignment>> {
+  typedef support::detail::packed_endian_specific_integral<value_type, endian,
+                                                           alignment>
+      endian_type;
+
+  static void output(const endian_type &E, void *Ctx,
+                     llvm::raw_ostream &Stream) {
+    ScalarTraits<value_type>::output(static_cast<value_type>(E), Ctx, Stream);
+  }
+  static StringRef input(StringRef Str, void *Ctx, endian_type &E) {
+    value_type V;
+    auto R = ScalarTraits<value_type>::input(Str, Ctx, V);
+    E = static_cast<endian_type>(V);
+    return R;
+  }
+
+  static bool mustQuote(StringRef Str) {
+    return ScalarTraits<value_type>::mustQuote(Str);
+  }
+};
+
 // Utility for use within MappingTraits<>::mapping() method
 // to [de]normalize an object for use with YAML conversion.
 template <typename TNorm, typename TFinal>