[compiler-rt] Do not introduce __sanitizer namespace globally

The definitions in sanitizer_common may conflict with definitions from system headers because:

The runtime includes the system headers after the project headers (as per LLVM coding guidelines).
lib/sanitizer_common/sanitizer_internal_defs.h pollutes the namespace of everything defined after it, which is all/most of the sanitizer .h and .cc files and the included system headers with: using namespace __sanitizer; // NOLINT
This patch solves the problem by introducing the namespace only within the sanitizer namespaces as proposed by Dmitry.

Differential Revision: https://reviews.llvm.org/D21947

llvm-svn: 281657