[llvm-symbolizer] Allow single letter command flags grouping.

Summary: Currently llvm-symbolizer doesn't allow flags combining. This patch allows such grouping behavior just like addr2line. Motivation: https://bugs.llvm.org/show_bug.cgi?id=40304

Reviewers: jhenderson, ruiu

Reviewed By: jhenderson

Subscribers: rupprecht, llvm-commits

Differential Revision: https://reviews.llvm.org/D57046

llvm-svn: 351936