Rename 'intrinsic' to 'builtin'

This matches the term used in the WGSL spec.

Change-Id: I4603332b828450c126ef806f1064ed54f372013f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/78787
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/builtins/arrayLength/via_let_no_struct.wgsl

@@ -0,0 +1,8 @@
+@group(0) @binding(0) var<storage, read> G : array<i32>;
+
+@stage(compute) @workgroup_size(1)
+fn main() {
+    let p = &G;
+    let p2 = p;
+    let l1 : u32 = arrayLength(p2);
+}