Thread safety analysis: split warnings into two groups: attribute warnings

Thread safety analysis: split warnings into two groups: attribute warnings
which are checked in the parser, and analysis warnings that require the
full analysis.  This allows attribute syntax to be checked independently
of the full thread safety analysis.  Also introduces a new warning for the
case where a string is used as a lock expression; this allows the analysis
to gracefully handle expressions that would otherwise cause a parse error.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155129 91177308-0d34-0410-b5e6-96231b3b80d8