While one can disable building the embedded stdlib, the tests for that
feature were unconditionally added. If one wants to just build the
compiler without any stdlib (except target), a bunch of embedded tests
would have failed.
Inject the value of `SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB` into the Lit
test system as a feature `embedded_stdlib` and add `REQUIRES:` to
a couple of tests outside the `embedded/` directory that seems to use
the experimental feature. Make the tests in `embedded/` unsupported in
the local Lit configuration file.
To properly initialize the compiler instance on LLDB when debugging
embedded Swift programs, emit "-enable-embedded-swift" into the
DW_AT_APPLE_flags attribute of the compile unit.
