[analyzer] After CFG has been linearized, we can have a situation where an...

[analyzer] After CFG has been linearized, we can have a situation where an ExpoledNode has an invalid SourceLocation (which has no correspondence in the source code). This commit is the first step to solve this problem.
 - It adds LocationContext to the PathDiagnosticLocation object and uses it to lookup the enclosing statement with a valid location.
 - So far, the LocationContext is only available when the object is constructed from the ExplodedNode.
 - Already found some subtle bugs(in plist-output-alternate.m) where the intermediate diagnostic steps were not previously shown. 

llvm-svn: 139703