Made SkSL DSL into public API

In addition to the unsurprising changes to eliminate references to
src/, we also had to tighten up some C++17-isms as they are not
permitted in public headers.

Change-Id: Ie5005a33d7a135e69fb66beca5e7a5f960dbd453
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/378496
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/src/sksl/SkSLASTNode.cpp b/src/sksl/SkSLASTNode.cpp
index e25bdf0..55a2cee 100644
--- a/src/sksl/SkSLASTNode.cpp
+++ b/src/sksl/SkSLASTNode.cpp
@@ -5,9 +5,9 @@
  * found in the LICENSE file.
  */
 
+#include "include/private/SkSLString.h"
 #include "src/sksl/SkSLASTNode.h"
 #include "src/sksl/SkSLOperators.h"
-#include "src/sksl/SkSLString.h"
 
 namespace SkSL {