Freshly built libc++ introduces issues when building `swift-build`:
```
Undefined symbols for architecture arm64:
"std::__1::__hash_memory(void const*, unsigned long)", referenced from:
(anonymous namespace)::CAPIBuildDB::buildUpKeyCache(std::__1::vector<llbuild::core::KeyType, std::__1::allocator<llbuild::core::KeyType>>&) in BuildDB-C-API.cpp.o
_llb_build_key_make in BuildKey-C-API.cpp.o
_llb_build_key_make_command in BuildKey-C-API.cpp.o
_llb_build_key_make_custom_task in BuildKey-C-API.cpp.o
_llb_build_key_make_custom_task_with_data in BuildKey-C-API.cpp.o
_llb_build_key_make_directory_contents in BuildKey-C-API.cpp.o
_llb_build_key_make_filtered_directory_contents in BuildKey-C-API.cpp.o
...
ld: symbol(s) not found for architecture arm64
```
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Swift SDKs require Swift SDK Generator to be built with its dependencies. Additionally, now that tests for Wasm stdlib run by default, we should include required tools. Embedded Swift build is also enabled in the documented invocation.
Only general intro and the "Relative Pointers" section are added to get started. More sections to be added in the future after preliminary feedback on this document is gathered.
Related to SR-9307
Max Desiatov
Yuta Saito
Max Desiatov