Pico-optimize `SkipWhitespace` to use `StripLeadingAsciiWhitespace`.

The `SkipWhitespace` predicate in `absl::StrSplit` now uses `absl::StripLeadingAsciiWhitespace` instead of `absl::StripAsciiWhitespace`.

Since the predicate only checks if the string is empty after stripping, and both functions reduce strings consisting only of whitespace to empty, this change does not alter the filtering behavior of `SkipWhitespace`, but spares one redundant call to `absl::StripTrailingAsciiWhitespace`.

PiperOrigin-RevId: 832467122
Change-Id: I5bf1ed99ded039d7070111d2f0cb50bc8b3d701a

absl/strings/str_split.h

diff --git a/absl/strings/str_split.h b/absl/strings/str_split.h
index cf53ccf..29fa4f7 100644
--- a/absl/strings/str_split.h
+++ b/absl/strings/str_split.h
@@ -382,7 +382,7 @@
 //   // v[0] == " a ", v[1] == " ", v[2] == "b"
 struct SkipWhitespace {
   bool operator()(absl::string_view sp) const {
-    sp = absl::StripAsciiWhitespace(sp);
+    sp = absl::StripLeadingAsciiWhitespace(sp);
     return !sp.empty();
   }
 };