Remove InitForm::index (#6817)

This ensures that equal forms always have equal representations (because
the index depends on how the form is used, not on the value of the form
itself).

As a byproduct, also remove `NextCallParamIndex`.

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Nicholas Bishop <nicholasbishop@google.com>
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Co-authored-by: Dana Jansens <danakj@orodu.net>
Co-authored-by: Boaz Brickner <brickner@google.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>
Co-authored-by: MK4070 <60286678+MK4070@users.noreply.github.com>
Co-authored-by: Christopher Di Bella <cjdb@google.com>

toolchain/check/full_pattern_stack.h

@@ -46,7 +46,6 @@ class FullPatternStack {
   auto PushFullPattern(Kind kind) -> void {
     kind_stack_.push_back(kind);
     bind_name_stack_.PushArray();
-    param_index_stack_.push_back(SemIR::CallParamIndex(0));
   }
 
   // Marks the end of an implicit parameter list, and the presumptive start
@@ -88,7 +87,6 @@ class FullPatternStack {
   auto PopFullPattern() -> void {
     kind_stack_.pop_back();
     bind_name_stack_.PopArray();
-    param_index_stack_.pop_back();
   }
 
   // Records that `name_id` was introduced by the current full-pattern.
@@ -104,13 +102,6 @@ class FullPatternStack {
                  kind_stack_.size());
   }
 
-  // Allocates the next unallocated CallParamIndex, starting from 0.
-  auto NextCallParamIndex() -> SemIR::CallParamIndex {
-    auto result = param_index_stack_.back();
-    ++param_index_stack_.back().index;
-    return result;
-  }
-
  private:
   LexicalLookup* lookup_;
 
@@ -121,8 +112,6 @@ class FullPatternStack {
     SemIR::InstId inst_id;
   };
   ArrayStack<LookupEntry> bind_name_stack_;
-
-  llvm::SmallVector<SemIR::CallParamIndex> param_index_stack_;
 };
 
 }  // namespace Carbon::Check