[tsan] Properly describe GCD worker threads in reports

When dealing with GCD worker threads, TSan currently prints weird things like "created by thread T-1" and "[failed to restore the stack]" in reports. This patch avoids that and instead prints "Thread T3 (...) is a GCD worker thread".

Differential Revision: https://reviews.llvm.org/D29103

llvm-svn: 293882