Implement faux-body-synthesis of well-known functions in the static analyzer when

their implementations are unavailable.  Start by simulating dispatch_sync().

This change is largely a bunch of plumbing around something very simple.  We
use AnalysisDeclContext to conjure up a fake function body (using the
current ASTContext) when one does not exist.  This is controlled
under the analyzer-config option "faux-bodies", which is off by default.

The plumbing in this patch is largely to pass the necessary machinery
around.  CallEvent needs the AnalysisDeclContextManager to get
the function definition, as one may get conjured up lazily.

BugReporter and PathDiagnosticLocation needed to be relaxed to handle
invalid locations, as the conjured body has no real source locations.
We do some primitive recovery in diagnostic generation to generate
some reasonable locations (for arrows and events), but it can be
improved.

llvm-svn: 164339