[libc][bazel] Add support for generating headers in Bazel

[vonosmas]

Currently, there's no supported way to invoke hdrgen tool from Bazel build. In order to support generating headers in Bazel build we need to:

• Add integration with pyyaml to Bazel build. Otherwise, Bazel-build libc:hdrgen py_binary can't run, since it fails in "import yaml" statement.
• Add libc_generated_header macro to wrap genrule that would invoke hdrgen tool to generate public libc header from YAML file.
• Use libc_generated_header for some of the common headers important for downstream users.
• Add support for running header tests: libc/test/include has a lot of unit tests, which are not ported to Bazel currently.
• (optional) add a convenient way to group generated headers, expose them together, and let llvm-libc users depend on them, similar to how libc_release_library is used to expose a subset of llvm-libc functions.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Alexey Samsonov (vonosmas)

Currently, there's no supported way to invoke hdrgen tool from Bazel build. In order to support generating headers in Bazel build we need to:

• Add integration with pyyaml to Bazel build. Otherwise, Bazel-build libc:hdrgen py_binary can't run, since it fails in "import yaml" statement.
• Add libc_generated_header macro to wrap genrule that would invoke hdrgen tool to generate public libc header from YAML file.
• Use libc_generated_header for some of the common headers important for downstream users.
• Add support for running header tests: libc/test/include has a lot of unit tests, which are not ported to Bazel currently.
• (optional) add a convenient way to group generated headers, expose them together, and let llvm-libc users depend on them, similar to how libc_release_library is used to expose a subset of llvm-libc functions.