Since libDemangling is included in the Swift standard library,
ODR violations can occur on platforms that allow statically
linking stdlib if Swift code is linked with other compiler
libraries that also transitively pull in libDemangling, and if
the stdlib version and compiler version do not match exactly
(even down to commit drift between releases). This lets the
runtime conditionally segregate its copies of the libDemangling
symbols from those in the compiler using an inline namespace
without affecting usage throughout source.
Separate out the host build of SwiftRemoteMirror from the "target" build
(the host for the standard library may be different from the host for
the compiler). Restructure the build to ensure that we use the correct
compiler for building the SwiftRemoteMirror for the host. This fixes a
build issue when building for Linux AArch64.
