DWARFFormValue.cpp: Appease gcc to give explicit constructors.

  error: conversion from `const uint8_t*' to non-scalar type `llvm::ArrayRef<unsigned char>' requested

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193575 91177308-0d34-0410-b5e6-96231b3b80d8