[libc++] Avoid triggering warnings for implicit conversion

This started as fixing a typo in a ADDITIONAL_COMPILE_FLAGS directive
which turned out to uncover a few places where we warned about signedness
changes.

As a fly-by fix, this updates the various __advance overloads
for style consistency.

Differential Revision: https://reviews.llvm.org/D106372

NOKEYCHECK=True
GitOrigin-RevId: f86570cd52419562b4c528c6e8a05ced00450c16
diff --git a/include/tuple b/include/tuple
index 032ac86..e1019ef 100644
--- a/include/tuple
+++ b/include/tuple
@@ -1165,7 +1165,7 @@
 
 namespace __find_detail {
 
-static constexpr size_t __not_found = -1;
+static constexpr size_t __not_found = static_cast<size_t>(-1);
 static constexpr size_t __ambiguous = __not_found - 1;
 
 inline _LIBCPP_INLINE_VISIBILITY