Add builtins for compound assignment operators. (#5335)

Provide builtins for compound assignments instead of defining them in
the prelude as a use of a binary operator and an assignment. This allows
us to lower compound assignment directly to LLVM operations instead of
producing a function call. In the short term this also allows us to
define a type-generic compound assignment in the prelude.

toolchain/lower/function_context.cpp

@@ -7,6 +7,7 @@
 #include "common/vlog.h"
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/sem_ir/file.h"
+#include "toolchain/sem_ir/generic.h"
 
 namespace Carbon::Lower {
 
@@ -171,6 +172,10 @@ auto FunctionContext::FinishInit(SemIR::TypeId type_id, SemIR::InstId dest_id,
   }
 }
 
+auto FunctionContext::GetTypeOfInst(SemIR::InstId inst_id) -> SemIR::TypeId {
+  return SemIR::GetTypeOfInstInSpecific(sem_ir(), specific_id(), inst_id);
+}
+
 auto FunctionContext::CopyValue(SemIR::TypeId type_id, SemIR::InstId source_id,
                                 SemIR::InstId dest_id) -> void {
   switch (auto rep = SemIR::ValueRepr::ForType(sem_ir(), type_id); rep.kind) {