cxa_demangle: fix -Wimplicit-fallthrough for GCC:7

Use the C++11 (formalised in C++17) tag to indicate a fallthrough in the
switch case.  Silences a -Wimplicit-fallthrough warning with gcc:7

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@305173 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed