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/source/source_buffer.h

@@ -36,14 +36,13 @@ class SourceBuffer {
  public:
   // Opens and reads the contents of stdin. Returns a SourceBuffer on success.
   // Prints an error and returns nullopt on failure.
-  static auto CreateFromStdin(DiagnosticConsumer& consumer)
+  static auto MakeFromStdin(DiagnosticConsumer& consumer)
       -> std::optional<SourceBuffer>;
 
   // Opens the requested file. Returns a SourceBuffer on success. Prints an
   // error and returns nullopt on failure.
-  static auto CreateFromFile(llvm::vfs::FileSystem& fs,
-                             llvm::StringRef filename,
-                             DiagnosticConsumer& consumer)
+  static auto MakeFromFile(llvm::vfs::FileSystem& fs, llvm::StringRef filename,
+                           DiagnosticConsumer& consumer)
       -> std::optional<SourceBuffer>;
 
   // Use one of the factory functions above to create a source buffer.
@@ -60,7 +59,7 @@ class SourceBuffer {
  private:
   // Creates a `SourceBuffer` from the given `llvm::MemoryBuffer`. Prints an
   // error and returns nullopt on failure.
-  static auto CreateFromMemoryBuffer(
+  static auto MakeFromMemoryBuffer(
       llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> buffer,
       llvm::StringRef filename, bool is_regular_file,
       DiagnosticConsumer& consumer) -> std::optional<SourceBuffer>;