Is there no .pc file because there is no interest from the developpers or is it because no one volonteered to do it? In the latter case, I'll do it there is an interest to merge it in the source tree.

I came up with the following bash script that writes a pc file to stdout. The pc file addresses the LLVM shared object.

$ cat make-pkg-config.sh 
version=$(llvm-config --version)

echo Name: LLVM
echo Description: Low-level Virtual Machine compiler framework
# We keep the initial numeric part of a version number.
# I.e. a suffix like 'svn' or 'rc' will be removed.
# This way Cabal can match on version ranges.
echo Version: $(echo ${version} | sed 's/\([0-9.]\+\).*/\1/')
echo URL: http://www.llvm.org/
echo Requires:
echo Conflicts:
echo Libs: -L$(llvm-config --libdir) -lLLVM-${version}
echo Cflags: -I$(llvm-config --includedir)

I have written a Haskell program that performs almost the same as the given shell script:
  http://hackage.haskell.org/package/llvm-pkg-config

You may install it with
$ cabal install llvm-pkg-config