[Support] Workaround a GCC 4.8 bug on constant expression evaluation.
Summary:
- The following stripped code trigger a gcc-4.8 bug. To work that
around, move the alignment evaluation into template parameter.
```
// https://godbolt.org/z/58p5_X
//
enum { aligned = 0, unaligned = 1 };
template <typename T, int alignment> struct PickAlignment {
enum { value = alignment == 0 ? alignof(T) : alignment };
};
template <typename ValueType, std::size_t Alignment> struct packed {
private:
struct {
alignas(
PickAlignment<ValueType, Alignment>::value) char buffer[sizeof(int)];
} Value;
};
using ule16_t = packed<uint16_t, unaligned>;
ule16_t x;
```
- Also, replace `alignas` with `LLVMALIGN_AS` to improve the compiler
compatibility.
Reviewers: jfb
Subscribers: dexonsmith, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D65452
llvm-svn: 367329