Check if building on Android through the ANDROID_DATA environment variable, then set
SWIFT_ANDROID_NATIVE_SYSROOT to the default layout for the Termux app, and key all the
include, lib, and other SDK paths off of that. The system libc and a few other libraries
are linked against from /system/lib[64]. Finally, check if lit is running natively on
Android and don't use adb if so.
Extract the C++ header search path from the system header search path. This is
needed to ensure that we do not push the C++ headers into the swift compiler
search path. The reason here is that when building the module, we would attempt
to include the C++ headers, which we cannot do since the ClangImporter in swift
explicitly sets the langugage to C. This would break the stdlib build for
android.
ndk14 introduced "unified headers" which merged the headers for all
different API versions into one directory which effectively split the
"SWIFT_SDK_ANDROID_ARCH_${ARCH}_PATH" into two different directories.
Add include and library specific paths to various compilation and link
invocations across the Swift project to account for this change. Remove
some broken sysroot/sdk specific settings.
