Add Variable::fBuiltin, to track Variables owned by pre-includes

Similar to the same field on Enum and FunctionDeclaration, will be used
to facilitate cloning builtin variables into Programs that use them.

Bug: skia:10589
Change-Id: Ic63701c61ee4658a5ec72adb506cc96aa0b2836f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/321196
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
diff --git a/src/sksl/SkSLSPIRVCodeGenerator.cpp b/src/sksl/SkSLSPIRVCodeGenerator.cpp
index 38a58d8..81bcb1b 100644
--- a/src/sksl/SkSLSPIRVCodeGenerator.cpp
+++ b/src/sksl/SkSLSPIRVCodeGenerator.cpp
@@ -1875,6 +1875,7 @@
                                                    Modifiers(layout, Modifiers::kUniform_Flag),
                                                    name,
                                                    &intfStruct,
+                                                   /*builtin=*/false,
                                                    Variable::kGlobal_Storage));
                 InterfaceBlock intf(-1, intfVar, name, String(""),
                                     std::vector<std::unique_ptr<Expression>>(), st);