[python] Make the collections import future-proof
[python] Make the collections import future-proof

On Python 3.7 the old code raises a warning:

	DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    	class ArgumentsIterator(collections.Sequence):

On Python 3.8 it wouldn't work anymore.

Commited on behalf of Jakub Stasiak.

Differential Revision: https://reviews.llvm.org/D56341

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@350467 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang