[Driver] Pass -emit-irgen thru to frontend.

Previously, it was computing that the command to pass through was
-emit-ir based on the output file type.

Author: nate-chandler

lib/Driver/ToolChains.cpp

@@ -713,6 +713,9 @@ const char *ToolChain::JobContext::computeFrontendModeForCompile() const {
   case file_types::TY_SIB:
     return "-emit-sib";
   case file_types::TY_LLVM_IR:
+    if (Args.getLastArg(options::OPT_emit_irgen)) {
+      return "-emit-irgen";
+    }
     return "-emit-ir";
   case file_types::TY_LLVM_BC:
     return "-emit-bc";

test/Driver/emit-irgen.swift

@@ -0,0 +1,3 @@
+// RUN: %swiftc_driver -### -emit-irgen %s -o - | %FileCheck %s
+
+// CHECK: -emit-irgen