Introduce the ResolveLocationInAST function which takes an ASTContext and a...

Introduce the ResolveLocationInAST function which takes an ASTContext and a SourceLocation and it resolves it into a <Decl*, Stmt*> pair.

Decl* is the declaration associated with this source location and Stmt* is the statement/expression that the location points to.
If the location does not point to a statement node, Stmt* is null.

ResolveLocationInAST (along with converting a file:line:column triplet to a SourceLocation) will be useful for an IDE client and for clang's test suite.

llvm-svn: 74197