pgsql: Fix LLVM related headers to compile standalone (to fixcplusplus

Fix LLVM related headers to compile standalone (to fix cpluspluscheck).

Previously llvmjit.h #error'ed when USE_LLVM was not defined, to
prevent it from being included from code not having #ifdef USE_LLVM
guards - but that's not actually that useful after, during the
development of JIT support, LLVM related code was moved into a
separately compiled .so.  Having that #error means cpluspluscheck
doesn't work when llvm support isn't enabled, which isn't great.

Similarly add USE_LLVM guards to llvmjit_emit.h, and additionally make
sure it compiles standalone.

Per complaint from Tom Lane.

Author: Andres Freund
Discussion: https://postgr.es/m/19808.1548692361@sss.pgh.pa.us
Backpatch: 11, where JIT support was added

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/da05eb51debd5d4b0284bcafa728244183c303ae

Modified Files
--------------
src/include/jit/llvmjit.h      | 9 ++++++---
src/include/jit/llvmjit_emit.h | 9 +++++++++
2 files changed, 15 insertions(+), 3 deletions(-)