Thread-safety analysis: differentiate between two forms of analysis; a precise

Thread-safety analysis: differentiate between two forms of analysis; a precise
analysis that may give false positives because it is confused by aliasing, and
a less precise analysis that has fewer false positives, but may have false
negatives.  The more precise warnings are enabled by -Wthread-safety-precise.
An additional note clarify the warnings in the precise case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163537 91177308-0d34-0410-b5e6-96231b3b80d8