Remove MSVC workaround for AlignedCharArray (NFC)

[mehdi_amini]

Summary

The comment only mentioned "old version of MSVC".

[mehdi_amini]

(If someone can confirm it works on the current MSVC 2015)

[bkramer]

I tried this recently and it still doesn't work (rL284738)

[mehdi_amini]

Thanks bkramer! Let's just improve the comment then?

[bkramer]

Thanks! And sorry for missing this in my revert.

[rnk]

This whole type is basically a lie, though. 32-bit MSVC only ever aligns objects passed by value to 4 bytes. MSVC will issue an error if you try to pass something with __declspec(align) by value. This ifdef effectively works around that diagnostic, but fails to accomplish the goal of alignment.

If we ever depend on the alignment of these objects when they are passed by value, we are screwed. The only thing that saves us today is that we don't often pass highly aligned things by value.

So, there is more work to do here.

llvm/include/llvm/Support/AlignOf.h

 #include <cstddef>
 
 namespace llvm {
 
 /// \struct AlignedCharArray
 /// \brief Helper for building an aligned character array type.
 ///
 /// This template is used to explicitly build up a collection of aligned
-/// character array types. We have to build these up using a macro and explicit
-/// specialization to cope with old versions of MSVC where only an
-/// integer literal can be used to specify an alignment constraint. Once built
-/// up here, we can then begin to indirect between these using normal C++
-/// template parameters.
-
-// MSVC requires special handling here.
-#ifndef _MSC_VER
-
+/// character array types.
 template<std::size_t Alignment, std::size_t Size>
 struct AlignedCharArray {
   LLVM_ALIGNAS(Alignment) char buffer[Size];
 };
 
-#else // _MSC_VER
-
-/// \brief Create a type with an aligned char buffer.
-template<std::size_t Alignment, std::size_t Size>
-struct AlignedCharArray;
-
-// We provide special variations of this template for the most common
-// alignments because __declspec(align(...)) doesn't actually work when it is
-// a member of a by-value function argument in MSVC, even if the alignment
-// request is something reasonably like 8-byte or 16-byte. Note that we can't
-// even include the declspec with the union that forces the alignment because
-// MSVC warns on the existence of the declspec despite the union member forcing
-// proper alignment.
-
-template<std::size_t Size>
-struct AlignedCharArray<1, Size> {
-  union {
-    char aligned;
-    char buffer[Size];
-  };
-};
-
-template<std::size_t Size>
-struct AlignedCharArray<2, Size> {
-  union {
-    short aligned;
-    char buffer[Size];
-  };
-};
-
-template<std::size_t Size>
-struct AlignedCharArray<4, Size> {
-  union {
-    int aligned;
-    char buffer[Size];
-  };
-};
-
-template<std::size_t Size>
-struct AlignedCharArray<8, Size> {
-  union {
-    double aligned;
-    char buffer[Size];
-  };
-};
-
-
-// The rest of these are provided with a __declspec(align(...)) and we simply
-// can't pass them by-value as function arguments on MSVC.
-
-#define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
-  template<std::size_t Size> \
-  struct AlignedCharArray<x, Size> { \
-    __declspec(align(x)) char buffer[Size]; \
-  };
-
-LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16)
-LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32)
-LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64)
-LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128)
-
-#undef LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT
-
-#endif // _MSC_VER
-
 namespace detail {
 template <typename T1,
           typename T2 = char, typename T3 = char, typename T4 = char,
           typename T5 = char, typename T6 = char, typename T7 = char,
           typename T8 = char, typename T9 = char, typename T10 = char>
 class AlignerImpl {
   T1 t1; T2 t2; T3 t3; T4 t4; T5 t5; T6 t6; T7 t7; T8 t8; T9 t9; T10 t10;