llvm-build: Adopt generation of LLVM_LIBS_TO_EXPORT. Patch by

Brad King.

Move `LLVM_LIBS_TO_EXPORT` over to Makefile.llvmbuild and generate it
from `llvm-build` using the same logic used to export the dependencies
of these libraries.  This avoids depending on `llvm-config`.

This refactoring was originally motivated by issue #24154 due to commit
r243297 (Fix `llvm-config` to emit the linker flag for the combined
shared object, 2015-07-27) changing the output of `llvm-config --libs`
to not have the individual libraries when we configure with
`--enable-shared`.  That change was reverted by r244108 but this
refactoring makes sense on its own anyway.

llvm-svn: 245717