Add AST matchers for narrowing constructors that are default, copy, or move...

Add AST matchers for narrowing constructors that are default, copy, or move constructors, as well as functionality to determine whether a ctor initializer is a base initializer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244036 91177308-0d34-0410-b5e6-96231b3b80d8