Fix clang -Wimplicit-fallthrough warnings across llvm, NFC
This patch should not introduce any behavior changes. It consists of
mostly one of two changes:
- Replacing fall through comments with the LLVM_FALLTHROUGH macro
- Inserting 'break' before falling through into a case block consisting of only 'break'.
We were already using this warning with GCC, but its warning behaves
slightly differently. In this patch, the following differences are
- GCC recognizes comments that say "fall through" as annotations, clang doesn't
- GCC doesn't warn on "case N: foo(); default: break;", clang does
- GCC doesn't warn when the case contains a switch, but falls through the outer case.
I will enable the warning separately in a follow-up patch so that it can
be cleanly reverted if necessary.
Reviewers: alexfh, rsmith, lattner, rtrieu, EricWF, bollu
Differential Revision: https://reviews.llvm.org/D53950