Add cxxStdInitializerListExpr AST matcher

Summary:
This adds a new ASTMatcher for CXXStdInitializerListExprs that matches C++ initializer list expressions.

The primary motivation is to use it to fix [[ https://bugs.llvm.org/show_bug.cgi?id=32896 | PR32896 ]] (review here [[ https://reviews.llvm.org/D32767 | D32767 ]]).

Reviewers: alexfh, Prazek, aaron.ballman

Reviewed By: alexfh, aaron.ballman

Subscribers: malcolm.parsons, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D32810

llvm-svn: 302287