llvm: Bring back old expressions for llvm & clang 3.[123].
llvm: Enable also building of LLVM 3.[12] & Clang 3.[123].
clang_31: Allow clang to find system includes in gcc dirs.
Backported fix.
LLVM: Allow building gold plugins together with llvm 3.{1,2}.
Merge branch 'vlstill-llvm-multivers'