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