swift-compatibility-symbols, swift-def-to-strings-converter,
and swift-serialize-diagnostics don't use any Swift modules. But when
SWIFT_SWIFT_PARSER was enabled, they are linked with swiftCore. But
these binaries can be executed before the runtime is being built.
We need to stop them linking with swiftCore.
In Linux. Instead of setting temporary "fallback" RUNPATH, Set
LD_LIBRARY_PATH to builder's runtime when building standard library.
So we don't need to strip the temporary RUNPATH when installing.
Create a new diagnostics serialization library, splitting out of
swiftAST to reduce the overheads for building the tooling for
cross-compiling the toolchain. This should reduce the build time for
swift-serialize-diagnostics to enable cross-compilation.
