Fix clang -Wimplicit-fallthrough warnings across llvm, NFC

This patch should not introduce any behavior changes. It consists of
mostly one of two changes:
1. Replacing fall through comments with the LLVM_FALLTHROUGH macro
2. 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
relevant:
1. GCC recognizes comments that say "fall through" as annotations, clang
   doesn't
2. GCC doesn't warn on "case N: foo(); default: break;", clang does
3. 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

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@345882 91177308-0d34-0410-b5e6-96231b3b80d8

Author: rnk

lib/ReaderWriter/MachO/ArchHandler_x86_64.cpp

@@ -80,6 +80,7 @@ class ArchHandler_x86_64 : public ArchHandler {
     switch (ref->kindValue()) {
     case ripRel32Got:
       assert(targetNowGOT && "target must be GOT");
+      LLVM_FALLTHROUGH;
     case ripRel32GotLoad:
       const_cast<Reference *>(ref)
         ->setKindValue(targetNowGOT ? ripRel32 : ripRel32GotLoadNowLea);

lib/ReaderWriter/MachO/MachOLinkingContext.cpp

@@ -262,6 +262,7 @@ void MachOLinkingContext::configure(HeaderFileType type, Arch arch, OS os,
   case llvm::MachO::MH_OBJECT:
     _printRemainingUndefines = false;
     _allowRemainingUndefines = true;
+    break;
   default:
     break;
   }