Fix -Wimplicit-fallthrough warnings in regcomp.c

Summary:
Since clang does not support comment style fallthrough annotations
these should be switched.

Reviewers: aaron.ballman, nickdesaulniers, xbolva00

Reviewed By: aaron.ballman, nickdesaulniers, xbolva00

Subscribers: xbolva00, nickdesaulniers, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66487

llvm-svn: 369549