IndexSwiftASTWalker::initVarRefIndexSymbols wasn't handling getCurrentExpr() returning a nullptr
as it does when processing a reference to someVar in the below import:
import var SomeModule.someVar
This patch fixes rdar://problem/30118572 and adds tests for import var/func references.
We were implicitly assuming that a function reference could only happen
in an expression, ignoring the case of
import func Module.fooFunc
For now, this doesn't actually add the reference to the index because
initCallRefIndexSymbol doesn't allow references without a parent
expression. We can look at adding the reference, or maybe doing
something special to the import itself separately.
rdar://problem/26496135