[clang] Add AST matcher for initializer list members

[clang] Add AST matcher for initializer list members

Summary:
Much like hasArg for various call expressions, this allows LibTooling users to
match against a member of an initializer list.

This is currently being used as part of the abseil-duration-scale clang-tidy
check.

Differential Revision: https://reviews.llvm.org/D56090

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@350523 91177308-0d34-0410-b5e6-96231b3b80d8