Introduce ftl::ignore

`ftl::ignore` works like `std::ignore`, but one can also ignore multiple
values using `operator()`. `ftl::ignore(a, b);` is more compact than
`std::ignore = a; std::ignore = b;`, and conveys the same idea.

Bug: 185536303
Flag: EXEMPT New library code
Test: atest ftl_test

Change-Id: Ice549328eeffb90f59f7b902809def48f583d157
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index 08ce855..5244442 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -26,6 +26,7 @@
         "function_test.cpp",
         "future_test.cpp",
         "hash_test.cpp",
+        "ignore_test.cpp",
         "match_test.cpp",
         "mixins_test.cpp",
         "non_null_test.cpp",