[Clang] Prevent mlink-builtin-bitcode from internalizing the RPC client

Summary:
Currently, we only use `-mmlink-builtin-bitcode` for non-LTO NVIDIA
compiliations. THis has the problem that it will internalize the RPC
client symbol which needs to be visible to the host. To counteract that,
I put `retain` on it, but this also prevents optimizations on the global
itself, so the passes we have that remove the symbol don't work on
OpenMP anymore. This patch does the dumbest solution, adding a special
string check for it in clang. Not the best solution, the runner up would
be to habe a clang attribute for `externally_inititliazed` because those
can't be internalized, but that might have some unfortunate
side-effects. Alternatively we could make NVIDIA compilations do LTO all
the time, but that would affect some users and it's harder than I
thought.

Author: jhuber6

clang/lib/CodeGen/CodeGenAction.cpp

@@ -246,7 +246,8 @@ bool BackendConsumer::LinkInModules(llvm::Module *M) {
           *M, std::move(LM.Module), LM.LinkFlags,
           [](llvm::Module &M, const llvm::StringSet<> &GVS) {
             internalizeModule(M, [&GVS](const llvm::GlobalValue &GV) {
-              return !GV.hasName() || (GVS.count(GV.getName()) == 0);
+              return !GV.hasName() || (GVS.count(GV.getName()) == 0) ||
+                     GV.getName().starts_with("__llvm_rpc_client");
             });
           });
     } else

offload/DeviceRTL/src/Misc.cpp

@@ -113,10 +113,8 @@ void *indirectCallLookup(void *HstPtr) {
 }
 
 /// The openmp client instance used to communicate with the server.
-/// FIXME: This is marked as 'retain' so that it is not removed via
-/// `-mlink-builtin-bitcode`
-[[gnu::visibility("protected"), gnu::weak,
-  gnu::retain]] rpc::Client Client asm("__llvm_rpc_client");
+[[gnu::visibility("protected"),
+  gnu::weak]] rpc::Client Client asm("__llvm_rpc_client");
 
 } // namespace impl
 } // namespace ompx