Migrate FunctionCall conversion out of IRGenerator.

We don't currently do FunctionCall optimization, but implementing
something along the lines of skia:10835 would probably involve doing
rewrites for optimization in FunctionCall::Make. This CL is the first
step down that road.

Change-Id: I249b02412e7ebac21bb98d6c5d61af3dcd6f1e69
Bug: skia:11342
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/387156
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/gn/sksl.gni b/gn/sksl.gni
index 582e176..f4f7a1a 100644
--- a/gn/sksl.gni
+++ b/gn/sksl.gni
@@ -111,6 +111,7 @@
   "$_src/sksl/ir/SkSLFloatLiteral.h",
   "$_src/sksl/ir/SkSLForStatement.cpp",
   "$_src/sksl/ir/SkSLForStatement.h",
+  "$_src/sksl/ir/SkSLFunctionCall.cpp",
   "$_src/sksl/ir/SkSLFunctionCall.h",
   "$_src/sksl/ir/SkSLFunctionDeclaration.h",
   "$_src/sksl/ir/SkSLFunctionDefinition.h",