ProgramBuilder: Don't wrap call-exprs with a call-stmts

ast::CallExpression will soon encompase function & intrinsic calls,
along with type-constructors and type-casts. The latter two cannot be
wrapped with a CallStatement, so change
ProgramBuilder::WrapInStatement() to always assign the expression to a
temporary.

Fix the few places that actually relied on this behavior to use
CallStmt() explicitly.

Bug: tint:888
Change-Id: I48b8a2be73128df9cd2b4bdcc00ae81c4a872359
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/69104
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/resolver/resolver_test.cc

@@ -913,8 +913,8 @@ TEST_F(ResolverTest, Function_CallSites) {
   auto* call_2 = Call("foo");
   auto* bar = Func("bar", ast::VariableList{}, ty.void_(),
                    {
-                       WrapInStatement(call_1),
-                       WrapInStatement(call_2),
+                       CallStmt(call_1),
+                       CallStmt(call_2),
                    });
 
   EXPECT_TRUE(r()->Resolve()) << r()->error();