[ADT] add StringViewExtras llvm::starts_with for std::string_view

std::string_view::starts_with isn't available until C++20. Create
llvm::starts_with for now; we can delete this when LLVM moves to C++20
one day.

To run the newly added unit test:
$ cd llvm/build; ninja ADTTests; cd -
$ ./llvm/build/unittests/ADT/ADTTests --gtest_filter=StringViewExtrasTest.\*

Reviewed By: MaskRay, erichkeane

Differential Revision: https://reviews.llvm.org/D148367