[android] conditionally build DS2 for the Android SDK

[andrurogerz]

Conditionally include the DS2debug server in the Android Swift SDK build. DS2 and its dependencies are only included when build.ps1 is invoked with the -IncludeDS2 flag.

This change is comprised of three primary parts:

1. Fetch and unpack WinFlexBison tools; flex and bison equivalents are required to build DS2.
2. Build the RegsGen2 host tool, specifying the previously unpacked win_flex.exe and win_bison.exe tools.
3. Build ds2 binary for each each requested Android SDK architecture and install it to the proper location under the Android.platform directory.

NOTE: the Android SDK isn't currently included in the installer build, so the ds2 binary only ends up in the output directory and not in any .msi file.

Test Plan

Build with and without ds2 included

cd S:\SourceCache
.\swift\utils\build.cmd -AndroidSDKs aarch64,x86_64
.\swift\utils\build.cmd -AndroidSDKs aarch64,x86_64 -IncludeDS2

Confirm the ds2 binary appears in the expected location only when -IncludeDS2 is specified

cd S:\b
dir /s /b ds2
S:\b\aarch64\Android\ds2
S:\b\aarch64\Android\ds2\ds2
S:\b\arm64\Android.platform\Developer\Library\aarch64-unknown-linux-android\bin\ds2
S:\b\x64\Android.platform\Developer\Library\x86_64-unknown-linux-android\bin\ds2
S:\b\x86_64\Android\ds2
S:\b\x86_64\Android\ds2\ds2

[andrurogerz]

@hjyamauchi would you mind having a look at this change?

@shahmishal I believe you have some stake in build.ps1 changes not breaking CI. This change is meant to have no effect unless the new argument is provided, so should be a noop for you.

[hjyamauchi]

LGTM. Note the IncludeDS2 flag is an opt-in (off by default.)

[hjyamauchi]

@swift-ci please test

[compnerd]

@swift-ci please smoke test