Support the 'a' scanf length modifier as an extension in C++.

It should not be supported in C++11, since that uses the C99 standard
library, in which 'a' is a format specifier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147310 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed