[Clang] Enable opaque pointers by default

Enable opaque pointers by default in clang, which can be disabled
either via cc1 option -no-opaque-pointers or cmake flag
-DCLANG_ENABLE_OPAQUE_POINTERS=OFF.

See https://llvm.org/docs/OpaquePointers.html for context.

Differential Revision: https://reviews.llvm.org/D123300

clang/CMakeLists.txt

@@ -251,7 +251,7 @@ option(CLANG_DEFAULT_PIE_ON_LINUX "Default to -fPIE and -pie on linux-gnu" ON)
 set(CLANG_ENABLE_OPAQUE_POINTERS "DEFAULT" CACHE STRING
     "Enable opaque pointers by default")
 if(CLANG_ENABLE_OPAQUE_POINTERS STREQUAL "DEFAULT")
-  set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL OFF)
+  set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL ON)
 elseif(CLANG_ENABLE_OPAQUE_POINTERS)
   set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL ON)
 else()

llvm/docs/OpaquePointers.rst

@@ -195,8 +195,10 @@ Transition State
 ================
 
 As of April 2022 both LLVM and Clang have complete support for opaque pointers,
-but typed pointers are still the default. Opaque pointers will be enabled by
-default in Clang soon.
+and opaque pointers are enabled by default in Clang. It is possible to
+temporarily restore the old default using the
+``-DCLANG_ENABLE_OPAQUE_POINTERS=OFF`` cmake option. Opaque pointers can be
+disabled for a single Clang invocation using ``-Xclang -no-opaque-pointers``.
 
 The MLIR and Polly monorepo projects are not fully compatible with opaque
 pointers yet.