transforms: Add a ProgramBuilder::Sym() helper

This creates a new, unnamed symbol.

Change-Id: I0ea11e812296c3215ba624a18777d3e7233e2ae4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/49441
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>

src/program_builder.h

@@ -878,6 +878,9 @@ class ProgramBuilder {
   // AST helper methods
   //////////////////////////////////////////////////////////////////////////////
 
+  /// @return a new unnamed symbol
+  Symbol Sym() { return Symbols().New(); }
+
   /// @param name the symbol string
   /// @return a Symbol with the given name
   Symbol Sym(const std::string& name) { return Symbols().Register(name); }