The implementation of [depr.c.headers] in D12747 introduced the necessary
C headers into libc++. This patch adds one more missing headers: limits.h
We spotted this due to a failing C++03 test [limits_h.pass.cpp] in our libc++
configuration; when the limits.h header is included from a C++ program, it now
bypassed the __config header and went directly into the underlying C library's
limits.h header, which is problematic for us because we use __config header to
configure the underlying C library's behaviour when used from a C++ context.