[ASTMatchers] Matchers for new[] operators

Two new matchers for `CXXNewExpr` are added which may be useful e.g. in
`clang-tidy` checkers. One of them is `isArray` which matches `new[]` but not
plain `new`. The other one, `hasArraySize` matches `new[]` for a given size.

llvm-svn: 318909