[Support] Add support for runtime endian values.

Endian functions only support reading and writing when the
endianness is known at compile time.  This patch adds overloads
where the endianness is a runtime value, and then delegates the
compile-time versions to the runtime versions.

Differential Revision: https://reviews.llvm.org/D29467

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294209 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed