configure.ac: Move LLVM functions to the top
configure.ac: Move LLVM functions to the top

This just moves code around so that all LLVM related stuff is at the
top of the file in the correct order.
No functional change.

Signed-off-by: Tobias Droste <6f0b7afb8e169bccf102c27c4328261728913687@gmx.de>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler