[sanitizer] Add early call handling to strlen interceptor

Summary:
The strlen interceptor is sometimes invoked too early for REAL(strlen) to
be initialized.  A special check is added to use internal_strlen for this
situation.

Reviewers: dim

Subscribers: llvm-commits, samsonov

Differential Revision: http://reviews.llvm.org/D18851

Change-Id: I3acc58f4abbae1904f25324abd84efea67aad0a2
llvm-svn: 265705