Updated GenLibDeps.pl to employ "use strict" to help prevent uses of variables

that have not yet been defined.

Removed used of grep and sed when parsing the results of "nm". This was
originally motivated because if the user has specified options to grep using
the environment variable GREP_OPTIONS this could break the script. Piping
through grep/sed/sort/uniq is also (to my understanding) not necessary, and
the equivalent operations can be done much faster in the Perl script.

Using a crude benchmark, these changes resulted in a 3x speedup in the
execution of GenLibDeps.pl.

llvm-svn: 44372