[tsan] Detect races on modifying accesses in Swift code

This patch allows the Swift compiler to emit calls to `__tsan_external_write` before starting any modifying access, which will cause TSan to detect races on arrays, dictionaries and other classes defined in non-instrumented modules. Races on collections from the Swift standard library and user-defined structs and a frequent cause of subtle bugs and it's important that TSan detects those on top of existing LLVM IR instrumentation, which already detects races in direct memory accesses.

Differential Revision: https://reviews.llvm.org/D31630

llvm-svn: 302050