Update LLVM build system for 249b40b558955afe5ac2b549edcf2d7f859c8cc9

This enables the use of modules for a significant build performance gain
when building with clang as host compiler or when using

Update LLVM build system for 249b40b558955afe5ac2b549edcf2d7f859c8cc9

This enables the use of modules for a significant build performance gain
when building with clang as host compiler or when using HAVE_LLVM=yes.
Switch libc++ to using the copy from the mono-repo.

show more ...