[ELF] Prevent crash with binary inputs with non-ascii file names

If using --format=binary with an input file name that has one or more non-ascii
characters in, LLD has undefined behaviour (it crashes on my Windows Debug build)
when calling isalnum with these non-ascii characters. Instead, of calling
std::isalnum, this patch uses an internal version that ignores the locale and
checks a specific subset of characters.

Reviewers: ruiu

Differential Revision: https://reviews.llvm.org/D37331

llvm-svn: 312705