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/parse/parse_fuzzer.cpp

@@ -29,7 +29,7 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
       llvm::MemoryBuffer::getMemBuffer(data_ref, /*BufferName=*/TestFileName,
                                        /*RequiresNullTerminator=*/false)));
   auto source =
-      SourceBuffer::CreateFromFile(fs, TestFileName, NullDiagnosticConsumer());
+      SourceBuffer::MakeFromFile(fs, TestFileName, NullDiagnosticConsumer());
 
   // Lex the input.
   SharedValueStores value_stores;