clang-tidy: add absl::string_view to stringy-types list

While absl isn't entirely green-light'ed in Chromium, checking uses of
it in cases where it is allowed to be used sounds useful, and we're
already running this check anyway, so... :)

Bug: b:187790543
Change-Id: I1596831486c5af5a5f940636ea9a46999ce7b70b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3742336
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1020857}

.clang-tidy

@@ -36,7 +36,7 @@
     - key:          bugprone-dangling-handle.HandleClasses
       value:        ::std::basic_string_view;::std::span;::absl::string_view;::base::BasicStringPiece;::base::span
     - key:          bugprone-string-constructor.StringNames
-      value:        ::std::basic_string;::std::basic_string_view;::base::BasicStringPiece
+      value:        ::std::basic_string;::std::basic_string_view;::base::BasicStringPiece;::absl::string_view
     - key:          modernize-use-default-member-init.UseAssignment
       value:        1
     # This relaxes modernize-use-emplace in some cases; we might want to make it