[libc++] Rename include/support to include/__support

We do ship those headers, so the directory name should not be something
that can potentially conflict with user-defined directories.

Differential Revision: https://reviews.llvm.org/D95956

GitOrigin-RevId: b51756819a85563ae063e98eeb3d6af8e44c8f64
diff --git a/include/bit b/include/bit
index fe36017..f8c37c3 100644
--- a/include/bit
+++ b/include/bit
@@ -62,7 +62,7 @@
 #include <__debug>
 
 #if defined(__IBMCPP__)
-#include "support/ibm/support.h"
+#include "__support/ibm/support.h"
 #endif
 #if defined(_LIBCPP_COMPILER_MSVC)
 #include <intrin.h>