[clang][modules] Move `UNHASHED_CONTROL_BLOCK` up in the AST file

When loading (transitively) imported AST file, `ModuleManager::addModule()` first checks it has the expected signature via `readASTFileSignature()`. The signature is part of `UNHASHED_CONTROL_BLOCK`, which is placed at the end of the AST file. This means that just to verify signature of an AST file, we need to skip over all top-level blocks, paging in the whole AST file from disk. This is pretty slow.

This patch moves `UNHASHED_CONTROL_BLOCK` to the start of the AST file, so that it can be read more efficiently. To achieve this, we use dummy signature when first emitting the unhashed control block, and then backpatch the real signature at the end of the serialization process.

This speeds up dependency scanning by over 9% and significantly reduces run-to-run variability of my benchmarks.

Depends on D158572.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D158573