[sanitizer] Scanf parser improvements.

Handle %a in cases when it is unambiguous.
Handle %m.

Patch by Jakub Jelinek.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@174882 91177308-0d34-0410-b5e6-96231b3b80d8
[sanitizer] Remove an extra va_copy.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@174883 91177308-0d34-0410-b5e6-96231b3b80d8