pgsql: backpatch "jit: Add support for LLVM 12."

backpatch "jit: Add support for LLVM 12."

As there haven't been problem on the buildfarm due to this change,
backpatch 6c57f2ed16e now.

Author: Andres Freund
Discussion: https://postgr.es/m/20201016011244.pmyvr3ee2gbzplq4@alap3.anarazel.de
Backpatch: 11-, where jit support was added

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fa7a52fb9943aa1c362a19fa886db284c82b62cd

Modified Files
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src/backend/jit/llvm/llvmjit.c   | 458 +++++++++++++++++++++++++++++++--------
src/tools/pgindent/typedefs.list |   1 +
2 files changed, 374 insertions(+), 85 deletions(-)