We are currently implementing support in LLDB that reconstructs the STL templates from
the target program in the expression evaluator. This reconstruction happens during the
import process from our debug info AST into the expression evaluation AST, which means
we need a way to intercept the ASTImporter import process in some controlled fashion.
This patch adds a import strategy to the ASTImporter that allows us to intercept any Import calls an
ASTImporter may make (either externally or while importing internally). We then provide
a strategy in LLDB that does do our template reconstruction.
 The reason for this is that we can't reliable import templates with the ASTImporter, so
actually reconstructing the template in the debug info AST and then importing it doesn't work.
It would also be much slower.