[ast-match] Add isAggregate and isFullyDesignated AST matchers

[hach-que]

These new AST matchers allow you to build a rule like this:

Rules:
- # Find initializers that are undesignated (requires C++20)
  Name: use-designated-init
  Matcher: |
    initListExpr(
      hasType(
        cxxRecordDecl(
          isAggregate(),
          matchesName("::Redpoint::EOS::API::*"),
          unless(hasAnyBase(hasType(cxxRecordDecl(has(fieldDecl())))))
        ).bind("type")
      ),
      unless(isFullyDesignated())
    ).bind("init")
  ErrorMessage: |
    initializer list should be fully designated
  Callsite: init

This detects any scenario where a value is initialized like SomeType{4, 5} when it should be initialized with SomeType{.A = 4, .B = 5} using the designated initializer list language feature of C++20.