pgsql: llvmjit: Work around bug in LLVM 3.9 causing crashes after 72559

llvmjit: Work around bug in LLVM 3.9 causing crashes after 72559438f92.

Unfortunately in LLVM 3.9 LLVMGetAttributeCountAtIndex(func, index)
crashes when called with an index that has 0 attributes. Since there's
no way to work around this in the C API, add a small C++ wrapper doing
so.

The only reason this didn't fail before 72559438f92 is that there
always are function attributes...

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20201016001254.w2nfj7gd74jmb5in@alap3.anarazel.de
Backpatch: 11-, like 72559438f92

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/efc9a8e9800cc04ff7460c3c78f9dc2120f3aea6

Modified Files
--------------
src/backend/jit/llvm/llvmjit.c        |  9 ++++++++-
src/backend/jit/llvm/llvmjit_wrap.cpp | 32 ++++++++++++++++++++++++++++++++
src/include/jit/llvmjit.h             |  2 ++
3 files changed, 42 insertions(+), 1 deletion(-)