Avoid looking up the same key repeatedly when the map has not changed.

Instead, look it up just once and hang on to the result for multiple
uses.  The "result" here might either be an iterator returned by
find() or a reference to the mapped-to value returned by at() or the
"[]" operator.
Add a "plugin" build target alias that just builds the LLVM plugin.
Merge branch 'interprocedural-analysis' of github.com:AlisaMaas/CArrayIntrospection into interprocedural-analysis