[ClangDiagnostics] Silence warning about fallthrough after PrintFatalError

Summary:
ClangDiagnosticsEmitter.cpp:1047:57: warning: this statement may fall through [-Wimplicit-fallthrough=]
       Builder.PrintFatalError("Unknown modifier type: " + Modifier);
                               ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
ClangDiagnosticsEmitter.cpp:1048:5: note: here
     case MT_Select: {
                   ^

Reviewers: rsmith, rtrieu

Reviewed By: rtrieu

Subscribers: rtrieu, ilya-biryukov, ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47340

llvm-svn: 333340