Place build.db under the scratch directory

[kateinoigakukun]

Motivation:

The build database should be shared between different triple builds to re-generate the build manifest correctly.

With the current implementation, the following build command sequence fails:

# 1st iteration: They both do not trigger "PackageStructure" build
$ swift build --experimental-swift-sdk wasm32-unknown-wasi
$ swift build

# 2nd iteration: They both trigger "PackageStructure" build because there is description.json and debug.yaml
$ swift build --experimental-swift-sdk wasm32-unknown-wasi
$ swift build

# 3rd iteration: Manifest cache entry in .build/wasm32-unknown-wasi/build.db created by 2nd iteration hits,
# so do not update debug.yaml. <--- Incorrect
$ swift build --experimental-swift-sdk wasm32-unknown-wasi

Modifications:

This changes the llbuild build database to be placed under .build/build.db instead of .build/<product triple>/build.db.
Also this change splits llbuild target names for each triple to avoid cache invalidation when switching triple.

Result:

build.db will be shared across product target triples, and SwiftPM will keep consistent cache state when switching triples.

[kateinoigakukun]

@swift-ci test