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/lower/function_context.cpp

@@ -71,7 +71,7 @@ auto FunctionContext::GetBlockArg(SemIR::InstBlockId block_id,
   return phi;
 }
 
-auto FunctionContext::CreateSyntheticBlock() -> llvm::BasicBlock* {
+auto FunctionContext::MakeSyntheticBlock() -> llvm::BasicBlock* {
   synthetic_block_ = llvm::BasicBlock::Create(llvm_context(), "", function_);
   return synthetic_block_;
 }