[asan] Refactor thread creation bookkeeping

Summary:
This is a pure refactoring change.  It paves the way for OS-specific
implementations, such as Fuchsia's, that can do most of the
per-thread bookkeeping work in the creator thread before the new
thread actually starts.  This model is simpler and cleaner, avoiding
some race issues that the interceptor code for thread creation has
to do for the existing OS-specific implementations.

Submitted on behalf of Roland McGrath.

Reviewers: vitalybuka, alekseyshl, kcc

Reviewed By: alekseyshl

Subscribers: phosek, filcab, llvm-commits, kubamracek

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36385

llvm-svn: 310432