[support] Add packed_endian_specific_integral::value() (#147974)

They are already implicitly convertible to the underlying type, but that
doesn't work in some contexts, and it can be useful to get the
underlying value without needing the remember/guess the right type.

I converted a couple of call sites to demonstrate usefulness, but
there's likely more of them. I know at least of at least a few in LLDB,
but I don't want to make this a cross-project patch.

Author: labath

llvm/include/llvm/Support/Endian.h

@@ -223,10 +223,11 @@ struct packed_endian_specific_integral {
 
   explicit packed_endian_specific_integral(value_type val) { *this = val; }
 
-  operator value_type() const {
+  value_type value() const {
     return endian::read<value_type, endian, alignment>(
       (const void*)Value.buffer);
   }
+  operator value_type() const { return value(); }
 
   void operator=(value_type newValue) {
     endian::write<value_type, endian, alignment>(

llvm/lib/ExecutionEngine/JITLink/aarch32.cpp

@@ -239,15 +239,14 @@ Error makeUnexpectedOpcodeError(const LinkGraph &G, const ThumbRelocation &R,
                                 Edge::Kind Kind) {
   return make_error<JITLinkError>(
       formatv("Invalid opcode [ {0:x4}, {1:x4} ] for relocation: {2}",
-              static_cast<uint16_t>(R.Hi), static_cast<uint16_t>(R.Lo),
-              G.getEdgeKindName(Kind)));
+              R.Hi.value(), R.Lo.value(), G.getEdgeKindName(Kind)));
 }
 
 Error makeUnexpectedOpcodeError(const LinkGraph &G, const ArmRelocation &R,
                                 Edge::Kind Kind) {
   return make_error<JITLinkError>(
-      formatv("Invalid opcode {0:x8} for relocation: {1}",
-              static_cast<uint32_t>(R.Wd), G.getEdgeKindName(Kind)));
+      formatv("Invalid opcode {0:x8} for relocation: {1}", R.Wd.value(),
+              G.getEdgeKindName(Kind)));
 }
 
 template <EdgeKind_aarch32 K> constexpr bool isArm() {

llvm/unittests/Support/EndianTest.cpp

@@ -237,6 +237,7 @@ TEST(Endian, PackedEndianSpecificIntegral) {
     reinterpret_cast<little32_t *>(little + 1);
 
   EXPECT_EQ(*big_val, *little_val);
+  EXPECT_EQ(big_val->value(), little_val->value());
 }
 
 TEST(Endian, PacketEndianSpecificIntegralAsEnum) {