Rename factory functions from 'Create' to 'Make' (#3706)

Similar to #3705, we actually have a mix of `Make` and `Create` in
factory functions too, so this PR is normalizing on `Make`. It's
intended to be consistent with the naming choice for Carbon factory
functions.

Note, MakeSyntheticBlock is the only one I feel a little weird about
because llvm's own APIs use Create, and this is essentially wrapping
LLVM calls. But the flipside is it also feels like a vague line to draw,
when we also differ from LLVM coding style in other ways.

toolchain/codegen/codegen.cpp

@@ -13,9 +13,8 @@
 
 namespace Carbon {
 
-auto CodeGen::Create(llvm::Module& module, llvm::StringRef target_triple,
-                     llvm::raw_pwrite_stream& errors)
-    -> std::optional<CodeGen> {
+auto CodeGen::Make(llvm::Module& module, llvm::StringRef target_triple,
+                   llvm::raw_pwrite_stream& errors) -> std::optional<CodeGen> {
   std::string error;
   const llvm::Target* target =
       llvm::TargetRegistry::lookupTarget(target_triple, error);