[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

llvm/lib/Transforms/IPO/Internalize.cpp

@@ -232,6 +232,8 @@ bool InternalizePass::internalizeModule(Module &M) {
     AlwaysPreserved.insert("__ssp_canary_word");
   else
     AlwaysPreserved.insert("__stack_chk_guard");
+  // For GPU host callbacks.
+  AlwaysPreserved.insert("__llvm_rpc_server");
 
   // Mark all functions not in the api as internal.
   IsWasm = Triple(M.getTargetTriple()).isOSBinFormatWasm();

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