ASTVector: Fix return value of various insert() methods.

Error caught using -fsanitize=pointer-overflow.

Expand ASTVectorTest to verify basic behavior,
test fails without functionality in this patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216385 91177308-0d34-0410-b5e6-96231b3b80d8