Add Java toolchains & platforms.

[iirina]

Add the required build_settings, config settings, constraint settings, constraint values, toolchain definitions and platforms that allow the majority of users to migrate their Java builds to the platform-based toolchain resolution. The PR implements the design described in Migrate Java toolchains to platforms. Define:

• a build_setting for each of: java language version, javac version, JDK version, remote JDK version. * for each build_setting, an associated constraint_setting (language, javac, jdk, remote_jdk) and all the supported constraint_values (e.g. language_level/ javac_version/ jdk_version/remote_jdk_version{8, 9, 10, 11, 12})
• config_settings for each valid combination of build_setting (flag) values {language_version, javac_version} and similarly a config_setting for each supported jdk_version
• a toolchain of javac_toolchain_type for each combination of supported platform, language level and javac version (e.g. linux_javac10_language8_toolchain)
• a toolchain of java_runtime type for each supported platform and remote JDK version (e.g. linux_remote_jdk10_runtime_toolchain)
• a local_jdk toolchain that points to @locak_jdk//:jdk and is compatible with HOST_CONSTRAINTS exposed by @local_config_platforms//:constraints.bzl
• a platform for each supported platform, using the config_settings define above to select the right constraint_values, depending on the chosen build_settings

[hlopko]

Looks sharp, thanks!

[iirina]

@hlopko Thanks for the review. I added more toolchains for java runtime, as well as for host javac and java runtime toolchains. PTAL.

[lberki]

Is this still relevant?

[MikhailTymchukFT]

Is this still relevant?

I double it. What is the current state of this?

[hlopko]

Closing for lack of progress.