hidl-gen: add FIXME to comments for -Lc++-impl

A lot of people are including/using this method unnecessarily. I'm
adding a note here so that people think about this before code
review.

Test: test_hidl_impl

Note, this is the test I use for simple changes like this:
function test_hidl_impl() {
    echo "Note: doing stuff in hardware/interfaces"
    hidl-gen -o hardware/interfaces/foo -Lc++-impl -r android.hardware:hardware/interfaces -r android.hidl:system/libhidl/transport android.hardware.tests.foo@1.0
    hidl-gen -o hardware/interfaces/foo -Landroidbp-impl -r android.hardware:hardware/interfaces -r android.hidl:system/libhidl/transport android.hardware.tests.foo@1.0
    sed hardware/interfaces/foo/Android.bp -i -e "s/-impl/-impl-asdf/g"
    m android.hardware.tests.foo@1.0-impl-asdf
}

Change-Id: I127e51238c3008f9b93ee1b8912641b133b98d87
diff --git a/generateCppImpl.cpp b/generateCppImpl.cpp
index 1ca899a..95b09a4 100644
--- a/generateCppImpl.cpp
+++ b/generateCppImpl.cpp
@@ -185,7 +185,8 @@
 
     out << "};\n\n";
 
-    out << "extern \"C\" "
+    out << "// FIXME: most likely delete, this is only for passthrough implementations\n"
+        << "// extern \"C\" "
         << ifaceName
         << "* ";
     generateFetchSymbol(out, ifaceName);
@@ -238,6 +239,7 @@
         return err;
     }
 
+    out.setLinePrefix("//");
     out << ifaceName
         << "* ";
     generateFetchSymbol(out, ifaceName);
@@ -246,6 +248,7 @@
     out << "return new " << baseName << "();\n";
     out.unindent();
     out << "}\n\n";
+    out.unsetLinePrefix();
 
     out << "}  // namespace implementation\n";
     enterLeaveNamespace(out, false /* leave */);