Revert r249929 ("Split <string.h> out of <cstring>").

This change caused problems when building code like povray that:
a) uses 'using namespace std;'
b) is built on an environment where the C library provides the "wrong"
   (non-const-correct) interface for the str* functions
c) makes an unqualified call to one of those str* functions

A patch is out for review to add a facility to fix this (and to give the
correct signatures for these functions whenever possible, even when the C
library does not do so). This revert is expected to be temporary.

llvm-svn: 251665