ast: Use IdentifierExpression for call targets

They are always identifiers, and this removes unnecessary type casting
from usages of CallExpression::func().

Change-Id: I7a11dc10658abab578395e20ad830cd5bc5a5b71
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51960
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/call_expression.cc

@@ -23,7 +23,7 @@ namespace ast {
 
 CallExpression::CallExpression(ProgramID program_id,
                                const Source& source,
-                               Expression* func,
+                               IdentifierExpression* func,
                                ExpressionList params)
     : Base(program_id, source), func_(func), params_(params) {
   TINT_ASSERT(func_);

src/ast/call_expression.h

@@ -20,6 +20,9 @@
 namespace tint {
 namespace ast {
 
+// Forward declarations.
+class IdentifierExpression;
+
 /// A call expression
 class CallExpression : public Castable<CallExpression, Expression> {
  public:
@@ -30,14 +33,14 @@ class CallExpression : public Castable<CallExpression, Expression> {
   /// @param params the parameters
   CallExpression(ProgramID program_id,
                  const Source& source,
-                 Expression* func,
+                 IdentifierExpression* func,
                  ExpressionList params);
   /// Move constructor
   CallExpression(CallExpression&&);
   ~CallExpression() override;
 
   /// @returns the func
-  Expression* func() const { return func_; }
+  IdentifierExpression* func() const { return func_; }
   /// @returns the parameters
   const ExpressionList& params() const { return params_; }
 
@@ -58,7 +61,7 @@ class CallExpression : public Castable<CallExpression, Expression> {
  private:
   CallExpression(const CallExpression&) = delete;
 
-  Expression* const func_;
+  IdentifierExpression* const func_;
   ExpressionList const params_;
 };