pgsql: Fix AC_REQUIRES breakage in LLVM autoconf tests.

Fix AC_REQUIRES breakage in LLVM autoconf tests. Any Autoconf macro that uses AC_REQUIRES -- directly or indirectly -- must not be inside a plain shell "if" test; if it is, whatever code gets pulled in by the AC_REQUIRES will also be inside that "if". Instead of "if" we can use AS_IF, which knows

pgsql: Fix AC_REQUIRES breakage in LLVM autoconf tests.

Fix AC_REQUIRES breakage in LLVM autoconf tests. Any Autoconf macro that uses AC_REQUIRES -- directly or indirectly -- must not be inside a plain shell "if" test; if it is, whatever code gets pulled in by the AC_REQUIRES will also be inside that "if". Instead of "if" we can use AS_IF, which knows