[ItaniumDemangle] reject A-F in FP literals (#82864)

The Itanium C++ ABI specifies that FP literals are encoded using a
lowercase hexadecimal string. Previously, libc++abi allowed uppercase
A-F characters but decoded them by subtracting 'a' from them, producing
negative digit values. It is especially confusing to accept an 'E' digit
because 'E' marks the end of the FP literal.

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -5541,7 +5541,7 @@ Node *AbstractManglingParser<Alloc, Derived>::parseFloatingLiteral() {
     return nullptr;
   std::string_view Data(First, N);
   for (char C : Data)
-    if (!std::isxdigit(C))
+    if (!(C >= '0' && C <= '9') && !(C >= 'a' && C <= 'f'))
       return nullptr;
   First += N;
   if (!consumeIf('E'))

libcxxabi/test/test_demangle.pass.cpp

@@ -30222,9 +30222,8 @@ struct FPLiteralCase {
      }},
 #endif
 #if LDBL_FP128
-    // This was found by libFuzzer+HWASan on aarch64 Android.
-    {"1\006ILeeeEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE",
-     {"\x6<-0x1.cecececececececececececececep+11983L>"}},
+    // A 32-character FP literal of long double type
+    {"3FooILeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeEE", {"Foo<-0x1.eeeeeeeeeeeeeeeeeeeeeeeeeeeep+12015L>"}},
 #endif
 };
 const unsigned NF = sizeof(fp_literal_cases) / sizeof(fp_literal_cases[0]);
@@ -30238,6 +30237,8 @@ const char* invalid_cases[] =
     "NSoERj5E=Y1[uM:ga",
     "Aon_PmKVPDk7?fg4XP5smMUL6;<WsI_mgbf23cCgsHbT<l8EE\0uVRkNOoXDrgdA4[8IU>Vl<>IL8ayHpiVDDDXTY;^o9;i",
     "_ZNSt16allocator_traitsISaIN4llvm3sys2fs18directory_iteratorEEE9constructIS3_IS3_EEEDTcl12_S_constructfp_fp0_spcl7forwardIT0_Efp1_EEERS4_PT_DpOS7_",
+    "3FooILdaaaaaaaaaaAAAAaaEE",
+    "3FooILdaaaaaaaaaaaaaaEE",
 #if !LDBL_FP80
     "_ZN5test01hIfEEvRAcvjplstT_Le4001a000000000000000E_c",
 #endif