Added the capability (turned off for now) to mark a

method as __attribute__ ((used)) when adding it to a
class.  This functionality is useful when stopped in
anonymous namespaces: expressions attached to classes
in anonymous namespaces are typically elided by Clang's
CodeGen because they have no namespaces are intended
not to be externally visible.  __attribute__ ((used))
forces CodeGen to emit the function.

Right now, __attribute__ ((used)) causes the JIT not to
emit the function, so we're not enabling it until we
fix that.

llvm-svn: 143469