Backport of llvmPackages_10 since Chromium 83 will depend on it.
See #83350 (comment)

llvmPackages_10: copy llvmPackages_9

* starting with rc2
* make `lldb` compilable again on Darwin
* separate out manpage creation for `lldb` into a new derivation
* minor tweaks to the patching of sources,
  some of which are backportable to earlier versions

(cherry picked from commit f111c6f)

llvmPackages_10: rc2 -> rc3

http://lists.llvm.org/pipermail/llvm-dev/2020-March/139729.html

Additionally cherry-picked 3 commits from `llvm-project/master`:
- llvm/llvm-project@d21664c
- llvm/llvm-project@3a0f6e6
- llvm/llvm-project@87dac7d

such that clang can automatically pick up the polly plugin from the
`llvm-polly` build.

(cherry picked from commit 3a84353)

llvmPackages_10: rc3 -> rc4

Only needed to update hashes and the version.
Updated comment for extension handling patch

(cherry picked from commit 0ec3f4e)

llvmPackages_10: removed extra polly-build

There is no good reason to have separate builds for polly and no-polly
versions. wwwwwThe reason for the two versions was (as far as I can
tell) to defer rebuilds (see ed60483).
Polly is now enabled by default.

(cherry picked from commit e9aa877)

llvmPackges_10: rc4 -> rc5

updated versino and hashes for new rc

(cherry picked from commit cdee144)

llvmPackages_10: rc5 -> release

updated hashes and url
updated comment regarding version/release_version to make it clearer

(cherry picked from commit 4665b2a)

[20.03] llvmPackages_10: init at 10.0.0