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 how to get this right
(cf commit 01051a987).

The only immediate problem from getting this wrong was that AC_PROG_AWK
had to be run twice, once inside the "if llvm" block and once in the
main line.  However, it broke a different patch I'm about to submit
more thoroughly.