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/tree_test.cpp

@@ -32,8 +32,8 @@ class TreeTest : public ::testing::Test {
   auto GetSourceBuffer(llvm::StringRef t) -> SourceBuffer& {
     CARBON_CHECK(fs_.addFile("test.carbon", /*ModificationTime=*/0,
                              llvm::MemoryBuffer::getMemBuffer(t)));
-    source_storage_.push_front(std::move(
-        *SourceBuffer::CreateFromFile(fs_, "test.carbon", consumer_)));
+    source_storage_.push_front(
+        std::move(*SourceBuffer::MakeFromFile(fs_, "test.carbon", consumer_)));
     return source_storage_.front();
   }