[Support] Rename llvm::support::endianness to llvm::endianness (#68174)

As part of an effort to make our codebase ready for the migration from
llvm::support::endianness to std::endian in C++20, this patch renames
llvm::support::endianness to llvm::endianness.

The intent of this patch is to make fully qualified names less
painful.  That is, with this patch, we can just say
llvm::endianness::big rather than llvm::support::endianness::big.

I'm not renaming llvm::support::endianness to llvm::endian because we
have a lot of places with "using namespace support;" where it would be
ambiguous whether "endian" refers to llvm::endian or
llvm::support::endian.

This patch defines several helpers for gradual migration:

  namespace llvm {
  namespace support {
  using endianness = llvm::endianness;
  constexpr llvm::endianness big = llvm::endianness::big;
  constexpr llvm::endianness little = llvm::endianness::little;
  constexpr llvm::endianness native = llvm::endianness::native;

While we are at it, this patch changes the enum to "enum class".  The
"enum class" prevents implicit conversions from endianness to bool.
I've fixed three such instances of implicit conversions:

  95f4b2a7
  8de2ecc2
  a7517e12