[Support] Remove MSVC specific AlignedCharArray implementation

[RKSimon]

Summary

Now that D64326 has landed we can remove the old MSVC AlignedCharArray implementations as VS2017+ can correctly handles alignas with by-value arguments.

This patch raises the question of whether we want to use llvm::AlignedCharArray directly in the codebase or whether some/all cases should be replaced with a type that uses alignas() directly?

[rnk]

I think the first version of MSVC 2017 doesn't allow you to pass aligned parameters when targeting x86, which has always been the problem child architecture:
https://gcc.godbolt.org/z/n_l_g5
Godbolt doesn't seem to have MSVC 2017 versions 19.11-19.13 to allow us to test.

LLVM claims it's minimum supported version is 19.11:
https://github.com/llvm/llvm-project/blob/456fc4fa6dc573c717f8db0d44a078f28060826e/llvm/cmake/modules/CheckCompilerVersion.cmake#L16

I think we need to check if 19.11 accepts this new code before we land this change, or we need to raise it the minimum to 19.14.

This patch raises the question of whether we want to use llvm::AlignedCharArray directly in the codebase or whether some/all cases should be replaced with a type that uses alignas() directly?

I think you're right, a lot of code can be simplified to use LLVM_ALIGNAS. I think we still need the LLVM_ALIGNAS portability macro until we raise our minimum GCC version, but it works more or less just as well as alignas.

[RKSimon]

This was handled in rL367282

include/llvm/Support/AlignOf.h

 #include <cstddef>
 
 namespace llvm {
 
 /// \struct AlignedCharArray
 /// 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 MSVC (at least till 2015) 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.
+/// character array types. 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
-
-template<std::size_t Alignment, std::size_t Size>
-struct AlignedCharArray {
+template <std::size_t Alignment, std::size_t Size> struct AlignedCharArray {
   alignas(Alignment) char buffer[Size];
 };
 
-#else // _MSC_VER
-
-/// 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;