Allow RecursiveASTVisitor to visit CXXCtorInitializer objects for which

isWritten() returns false, if shouldVisitImplicitCode() returns true.
Previously those CXXCtorInitializers were always skipped.

In order to make this change easier to test, this patch also extends the
test class template ExpectedLocationVisitor to support arbitrary numbers
of expected matches and disallowed matches.

llvm-svn: 162544