[StringExtras] Add a fromHex to complement toHex.

We already have a function toHex that will convert a string like
"\xFF\xFF" to the string "FFFF", but we do not have one that goes
the other way - i.e. to convert a textual string representing a
sequence of hexadecimal characters into the corresponding actual
bytes.  This patch adds such a function.

llvm-svn: 301356