Reapply r327151 (partially):

For our lldb customizations, instead of commenting out lines, use #ifdef
LLDB_ENABLE_ALL / #endif preprocess directives instead, so our diffs
against upstream only consi

Reapply r327151 (partially):

For our lldb customizations, instead of commenting out lines, use #ifdef
LLDB_ENABLE_ALL / #endif preprocess directives instead, so our diffs
against upstream only consist of added lines.

(Note that upstream has largely reshuffled the way optional lldb plugins
are handled, so we need a lot less of these #ifdefs. However, not all of
them can be dropped, unless we re-import several sources that we have
always skipped.)

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Merge llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and openmp
master 2e10b7a39b9, the last commit before the llvmorg-12-init tag, from
which release/11.x was branched.

Note that for now, I

Merge llvm, clang, compiler-rt, libc++, libunwind, lld, lldb and openmp
master 2e10b7a39b9, the last commit before the llvmorg-12-init tag, from
which release/11.x was branched.

Note that for now, I rolled back all our local changes to make merging
easier, and I will reapply the still-relevant ones after updating to
11.0.0-rc1.

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