Avoid a potential exit(1) in LLVMContext::diagnose() (llvm#84992)

by handling *all* errors in IRExecDiagnosticHandler. The function that
call this handles all unhandled errors with an `exit(1)`.

rdar://124459751

I don't really have a testcase for this, since the crash report I got
for this involved the Swift language plugin.

Author: adrian-prantl

lldb/source/Expression/IRExecutionUnit.cpp

@@ -212,18 +212,17 @@ struct IRExecDiagnosticHandler : public llvm::DiagnosticHandler {
   Status *err;
   IRExecDiagnosticHandler(Status *err) : err(err) {}
   bool handleDiagnostics(const llvm::DiagnosticInfo &DI) override {
-    if (DI.getKind() == llvm::DK_SrcMgr) {
+    if (DI.getSeverity() == llvm::DS_Error) {
       const auto &DISM = llvm::cast<llvm::DiagnosticInfoSrcMgr>(DI);
       if (err && err->Success()) {
         err->SetErrorToGenericError();
         err->SetErrorStringWithFormat(
-            "Inline assembly error: %s",
+            "IRExecution error: %s",
             DISM.getSMDiag().getMessage().str().c_str());
       }
-      return true;
     }
 
-    return false;
+    return true;
   }
 };
 } // namespace