SymbolTable: Change behavior of anonymous symbols

SymbolTable::New() used to build and return a symbol without a registered name. When you asked for the name of the symbol it would return tint_symbol_N, where N is the numerical identifier for the symbol.

This approach was a major tripping hazzard for  transforms that liked to fetch the source program name, and register it in the new program (in this situation, you should always use `CloneContext::Clone(Symbol)`).
Without special casing for unnamed symbols, you could end up promoting the unnamed symbol to a named symbol, and then colliding against a new unnamed symbol.
This is exactly what happened in tint:711.

Instead, with this change:

* The concept of unnamed symbols has been removed. All symbols  now have a name.
* The signature of `SymbolTable::New()` has been changed to take a name parameter (which defaults to 'tint_symbol'). This can be used to create a new, unique named symbol (possibly with a suffix), which will not collide with any existing symbols. Note these symbols may still collide if `SymbolTable::Register()` is called with the same name. All Transforms that currently use `SymbolTable::Register()` will be fixed in another change.
* The CloneContext has been updated to use `SymbolTable::New()` instead of `Register()`. This means that any symbols defined before a clone will not collide.
* `CloneContext::CloneSymbols()` has been added which allows a transform to pre-clone all the symbols from the source program. This can be used to avoid the authored identifiers being suffixed with a number, in the case a transform calls New() before the symbol is cloned.
* `Symbol::to_str()` has been changed to return `$<id>` instead of `tint_symbol_N`. This is to avoid any confusion between the actual name and the symbol ID.

Bug: tint:711
Bug: tint:712
Change-Id: I526e4b49b7027545613859de487e6a275686107a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/47631
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/reader/spirv/function_call_test.cc

@@ -44,16 +44,16 @@ TEST_F(SpvParserTest, EmitStatement_VoidCallNoParams) {
   ASSERT_TRUE(p->BuildAndParseInternalModule()) << p->error();
   const auto got = p->program().to_str(false);
   const char* expect = R"(Module{
-  Function tint_symbol_1 -> __void
+  Function $1 -> __void
   ()
   {
     Return{}
   }
-  Function tint_symbol_2 -> __void
+  Function $2 -> __void
   ()
   {
     Call[not set]{
-      Identifier[not set]{tint_symbol_1}
+      Identifier[not set]{$1}
       (
       )
     }