[llvm][ItaniumDemangle] Use __LDBL_MANT_DIG__ for configuring demangling of long doubles (#135968)

Syncing in the changes from
#134976 using the
`cp-to-llvm.sh` script.

Author: Michael137

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -38,8 +38,10 @@
 DEMANGLE_NAMESPACE_BEGIN
 
 template <class T, size_t N> class PODSmallVector {
-  static_assert(std::is_trivial<T>::value,
-                "T is required to be a trivial type");
+  static_assert(std::is_trivially_copyable<T>::value,
+                "T is required to be a trivially copyable type");
+  static_assert(std::is_trivially_default_constructible<T>::value,
+                "T is required to be trivially default constructible");
   T *First = nullptr;
   T *Last = nullptr;
   T *Cap = nullptr;
@@ -5739,14 +5741,16 @@ struct FloatData<double>
 template <>
 struct FloatData<long double>
 {
-#if defined(__mips__) && defined(__mips_n64) || defined(__aarch64__) || \
-    defined(__wasm__) || defined(__riscv) || defined(__loongarch__) || \
-    defined(__ve__)
-    static const size_t mangled_size = 32;
-#elif defined(__arm__) || defined(__mips__) || defined(__hexagon__)
-    static const size_t mangled_size = 16;
+#if __LDBL_MANT_DIG__ == 113 || __LDBL_MANT_DIG__ == 106
+  static const size_t mangled_size = 32;
+#elif __LDBL_MANT_DIG__ == 53 || defined(_MSC_VER)
+  // MSVC doesn't define __LDBL_MANT_DIG__, but it has long double equal to
+  // regular double on all current architectures.
+  static const size_t mangled_size = 16;
+#elif __LDBL_MANT_DIG__ == 64
+  static const size_t mangled_size = 20;
 #else
-    static const size_t mangled_size = 20;  // May need to be adjusted to 16 or 24 on other platforms
+#error Unknown size for __LDBL_MANT_DIG__
 #endif
     // `-0x1.ffffffffffffffffffffffffffffp+16383` + 'L' + '\0' == 42 bytes.
     // 28 'f's * 4 bits == 112 bits, which is the number of mantissa bits.