[libc++abi][ItaniumDemangle] Demangle DF16b as std::bfloat16_t (#120109)

This mangling is official in the Itanium C++ ABI specification and is
already supported by clang.

Author: frasercrmck

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -4330,9 +4330,12 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
     case 'h':
       First += 2;
       return make<NameType>("half");
-    //                ::= DF <number> _ # ISO/IEC TS 18661 binary floating point (N bits)
+    //       ::= DF16b         # C++23 std::bfloat16_t
+    //       ::= DF <number> _ # ISO/IEC TS 18661 binary floating point (N bits)
     case 'F': {
       First += 2;
+      if (consumeIf("16b"))
+        return make<NameType>("std::bfloat16_t");
       Node *DimensionNumber = make<NameType>(parseNumber());
       if (!DimensionNumber)
         return nullptr;

libcxxabi/test/test_demangle.pass.cpp

@@ -33,7 +33,7 @@
 // Is long double fp128?
 #define LDBL_FP128 (__LDBL_MANT_DIG__ == 113)
 
-const char *cases[][2] = {
+const char* cases[][2] = {
     // clang-format off
     {"_Z1A", "A"},
     {"_Z1Av", "A()"},
@@ -30245,6 +30245,8 @@ const char *cases[][2] = {
     {"_Z1fDSDRj", "f(_Sat unsigned _Fract)"},
     {"_Z1fDSDRl", "f(_Sat long _Fract)"},
     {"_Z1fDSDRm", "f(_Sat unsigned long _Fract)"},
+
+    {"_Z11bfloat16addDF16bDF16b", "bfloat16add(std::bfloat16_t, std::bfloat16_t)"},
     // clang-format on
 };
 

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -4330,9 +4330,12 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
     case 'h':
       First += 2;
       return make<NameType>("half");
-    //                ::= DF <number> _ # ISO/IEC TS 18661 binary floating point (N bits)
+    //       ::= DF16b         # C++23 std::bfloat16_t
+    //       ::= DF <number> _ # ISO/IEC TS 18661 binary floating point (N bits)
     case 'F': {
       First += 2;
+      if (consumeIf("16b"))
+        return make<NameType>("std::bfloat16_t");
       Node *DimensionNumber = make<NameType>(parseNumber());
       if (!DimensionNumber)
         return nullptr;