Migrate SkSL Inliner out of IRGenerator and into its own module.

The functional changes needed to make this migration possible have
already been submitted in prior CLs, so although this is large, it is
almost entirely a mechanical migration of code from one file to another.

Change-Id: I54380b52e38ebcec40ffbca7cb254f9655cb1865
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313909
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

Author: johnstiles-google

gn/sksl.gni

@@ -30,6 +30,8 @@ skia_sksl_sources = [
   "$_src/sksl/SkSLFileOutputStream.h",
   "$_src/sksl/SkSLIRGenerator.cpp",
   "$_src/sksl/SkSLIRGenerator.h",
+  "$_src/sksl/SkSLInliner.cpp",
+  "$_src/sksl/SkSLInliner.h",
   "$_src/sksl/SkSLLexer.cpp",
   "$_src/sksl/SkSLLexer.h",
   "$_src/sksl/SkSLMemoryLayout.h",