Flush diagnostics before printing semantics IR rather than after. (#3128)

This matches what we do for all other dump output.

No test: this is just changing the behavior of a dump mode, and is
really awkward to exercise without adding back in something like a lit
test to observe the behavior when stdout and stderr go to the same
place.

toolchain/driver/driver.cpp

@@ -449,6 +449,7 @@ auto Driver::Compile(const CompileOptions& options) -> bool {
     }
   }
   if (options.dump_semantics_ir) {
+    consumer->Flush();
     FormatSemanticsIR(tokenized_source, parse_tree, semantics_ir,
                       output_stream_);
   }