[llvm-cxxfilt] Replace isalnum with isAlnum from StringExtras

D104366 introduced a new llvm-cxxfilt test with non-ASCII characters,
which caused a failure on llvm-clang-x86_64-expensive-checks-win
builder, with a stack trace suggesting issue in a call to isalnum.

The argument to isalnum should be either EOF or a value that is
representable in the type unsigned char. The llvm-cxxfilt does not
perform a cast from char to unsigned char before the call, so the
value might be out of valid range.

Replace the call to isalnum with isAlnum from StringExtras, which takes
a char as the argument. This also makes the check independent of the
current locale.

Differential Revision: https://reviews.llvm.org/D110986