[llvm-symbolizer] Add -p as alias to -pretty-print

Summary: Provides -p as a short alias for -pretty-print. Motivation: https://bugs.llvm.org/show_bug.cgi?id=40076

Reviewers: samsonov, khemant, ruiu, rnk, fjricci, jhenderson

Reviewed By: jhenderson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56542

llvm-svn: 350832