[demangler] Avoid alignment warning

The alignment specified by a constant for the field
`BumpPointerAllocator::InitialBuffer` exceeded the alignment
guaranteed by `malloc` and `new` on Windows. This change set
the alignment value to that of `long double`, which is defined
by the used platform.

It fixes https://bugs.llvm.org/show_bug.cgi?id=37944.

Differential Revision: https://reviews.llvm.org/D48889

llvm-svn: 336311
diff --git a/llvm/lib/Demangle/ItaniumDemangle.cpp b/llvm/lib/Demangle/ItaniumDemangle.cpp
index 8dd182f..a2743bd 100644
--- a/llvm/lib/Demangle/ItaniumDemangle.cpp
+++ b/llvm/lib/Demangle/ItaniumDemangle.cpp
@@ -1950,7 +1950,7 @@
   static constexpr size_t AllocSize = 4096;
   static constexpr size_t UsableAllocSize = AllocSize - sizeof(BlockMeta);
 
-  alignas(16) char InitialBuffer[AllocSize];
+  alignas(long double) char InitialBuffer[AllocSize];
   BlockMeta* BlockList = nullptr;
 
   void grow() {