Description:
Currently

extra/clang is built with -DBUILD_SHARED_LIBS=ON (generate /usr/lib/libclangAST.so ... instead of libclangAST.a)
-DLLVM_LINK_LLVM_DYLIB=ON (link against libLLVM-7.so)

extra/llvm extra/lldb are built with -DBUILD_SHARED_LIBS=OFF (default) (generate /usr/lib/libLLVMSupport.a , not libLLVMSupport.so)

The discrepancy makes it easy to have collision of static constructors when the user links libLLVMSupport.a and libclangAST.so

# If the user builds https://github.com/MaskRay/ccls with cmake -H. -BRelease -DSYSTEM_CLANG=on -DLLVM_ENABLE_RTTI=on # no -DUSE_SHARED_LLVM=on
% ccls
CommandLine Error: Option 'help-list' registered more than once!

This is because both libLLVM-7.so (needed by libclang*.so) and libLLVMSupport.a define the static constructors of some cl::opt options (e.g. https://github.com/llvm-mirror/llvm/tree/master/lib/Support/CommandLine.cpp#L2039 HLOp "help-list")

I suggest unifying PKGBUILD of

* extra/llvm
* extra/clang (depends on llvm)
* extra/lldb (depends on both llvm and clang)

and using -DLLVM_BUILD_LLVM_DYLIB=ON -DUSE_SHARED_LIBS=ON for the unified build. (there will be libclangAST.so libLLVMSupport.so but no .a)

The separate build is also wasteful.

Note, -DLLVM_BUILD_LLVM_DYLIB=ON creates libLLVM-7.so, which is a unified .so containing all external symbols of libLLVM*.a or libLLVM*.so