Implement bitwise complement operator

This translates to/from OpNot for SPIR-V, and ~ for all three textual
language backends.

Fixed: tint:866
Change-Id: Id934fb309221e3fca0e7efa33edaaae137fd8085
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/54980
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/ast/unary_op.cc

@@ -23,6 +23,10 @@ std::ostream& operator<<(std::ostream& out, UnaryOp mod) {
       out << "address-of";
       break;
     }
+    case UnaryOp::kComplement: {
+      out << "complement";
+      break;
+    }
     case UnaryOp::kIndirection: {
       out << "indirection";
       break;

src/ast/unary_op.h

@@ -23,6 +23,7 @@ namespace ast {
 /// The unary op
 enum class UnaryOp {
   kAddressOf,    // &EXPR
+  kComplement,   // ~EXPR
   kIndirection,  // *EXPR
   kNegation,     // -EXPR
   kNot,          // !EXPR