cleanup: Fix all -Wsign-compare warnings.

.github/scripts/flags-clang.sh

@@ -36,9 +36,6 @@ add_flag -Wno-padded
 # This warns on things like _XOPEN_SOURCE, which we currently need (we
 # probably won't need these in the future).
 add_flag -Wno-reserved-id-macro
-# TODO(iphydf): Clean these up. They are likely not bugs, but still
-# potential issues and probably confusing.
-add_flag -Wno-sign-compare
 # We don't want to have default cases, we want to explicitly define all cases
 add_flag -Wno-switch-default
 # __attribute__((nonnull)) causes this warning on defensive null checks.

.github/scripts/flags-gcc.sh

@@ -47,8 +47,7 @@ add_flag -Wunused-value
 
 # struct Foo foo = {0}; is a common idiom.
 add_flag -Wno-missing-field-initializers
-# TODO(iphydf): Clean these up. They are likely not bugs, but still
-# potential issues and probably confusing.
+# Checked by clang, but gcc is warning when it's not necessary.
 add_flag -Wno-sign-compare
 # File transfer code has this.
 add_flag -Wno-type-limits