Migrate SkSLExternalFunction to the ir/ directory.

Change-Id: I8ca4fbba551624c61a4ebe7a4716750fe0b48196
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/395818
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index e9126d5..4fc104b 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -45,7 +45,6 @@
   "$_src/sksl/SkSLDehydrator.cpp",
   "$_src/sksl/SkSLDehydrator.h",
   "$_src/sksl/SkSLErrorReporter.h",
-  "$_src/sksl/SkSLExternalFunction.h",
   "$_src/sksl/SkSLFileOutputStream.h",
   "$_src/sksl/SkSLIRGenerator.cpp",
   "$_src/sksl/SkSLIRGenerator.h",
@@ -120,6 +119,7 @@
   "$_src/sksl/ir/SkSLExpressionStatement.cpp",
   "$_src/sksl/ir/SkSLExpressionStatement.h",
   "$_src/sksl/ir/SkSLExtension.h",
+  "$_src/sksl/ir/SkSLExternalFunction.h",
   "$_src/sksl/ir/SkSLExternalFunctionCall.h",
   "$_src/sksl/ir/SkSLExternalFunctionReference.h",
   "$_src/sksl/ir/SkSLField.h",