Fix const function-parameter assertion discovered by fuzzer.

During constant-folding, we baked in an assertion stating that any
const-typed variable reference ought to have an initial value, because
you can't declare a const variable without assigning a value. However,
function parameters are an exception to this rule! They are variable
references and are allowed to be const, but will not have an initial
value. (In this case, `const` just means you can't alter the value.)

In this case, all we needed to do was remove the assertion; we already
treated this case defensively and with the appropriate care.

Change-Id: I61242c6d08c59886c6992898f195771e6334f2b4
Bug: oss-fuzz:37465
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441239
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>

gn/sksl_tests.gni

@@ -98,6 +98,7 @@ sksl_error_tests = [
   "/sksl/errors/Ossfuzz32587.sksl",
   "/sksl/errors/Ossfuzz32851.sksl",
   "/sksl/errors/Ossfuzz37457.sksl",
+  "/sksl/errors/Ossfuzz37465.sksl",
   "/sksl/errors/OverflowFloatLiteral.sksl",
   "/sksl/errors/OverflowIntLiteral.sksl",
   "/sksl/errors/OverflowInt64Literal.sksl",

resources/sksl/errors/Ossfuzz37465.sksl

@@ -0,0 +1 @@
+float f(const float x) { 5+x; }

resources/sksl/shared/Functions.sksl

@@ -2,7 +2,7 @@
 
 uniform half4 colorGreen, colorRed;
 
-float foo(float2 v) {
+float foo(const float2 v) {
     return v.x * v.y;
 }
 

src/sksl/SkSLConstantFolder.cpp

@@ -213,8 +213,7 @@ const Expression* ConstantFolder::GetConstantValueForVariable(const Expression&
         }
         expr = var.initialValue();
         if (!expr) {
-            SkDEBUGFAILF("found a const variable without an initial value (%s)",
-                         var.description().c_str());
+            // Function parameters can be const but won't have an initial value.
             break;
         }
         if (expr->isCompileTimeConstant()) {

tests/sksl/errors/Ossfuzz37465.glsl

@@ -0,0 +1,4 @@
+### Compilation failed:
+
+error: 1: function 'f' can exit without returning a value
+1 error

tests/sksl/shared/Functions.glsl

@@ -2,7 +2,7 @@
 out vec4 sk_FragColor;
 uniform vec4 colorGreen;
 uniform vec4 colorRed;
-float foo_ff2(vec2 v) {
+float foo_ff2(const vec2 v) {
     return v.x * v.y;
 }
 void bar_vf(inout float x) {

tests/sksl/shared/Functions.metal

@@ -16,7 +16,7 @@ void _skOutParamHelper0_bar_vf(thread float& x) {
     bar_vf(_var0);
     x = _var0;
 }
-float foo_ff2(float2 v) {
+float foo_ff2(const float2 v) {
     return v.x * v.y;
 }
 void bar_vf(thread float& x) {