[lldb] Pass -fPIC flag even when DYLIB_ONLY is set

Summary:
It seems that when we rewrite a few rules to only build a dylib (i.e., when DYLIB_ONLY is set),
the rule for setting the CFLAGS for the dylib's object file compilation will no longer work. From what I can
see this is because in DYLIB_ONLY mode we pretend to compile the main executable so
the DYLIB_OBJECTS scope is actually never used.

This patch makes `-fPIC` unstopped if DYLIB_ONLY is set so that -fPIC actually ends up in the
CFLAGS for the dylib object file compilation.

The test for this is D80798 which only compiles on Linux with this patch.

Reviewers: friss, labath

Reviewed By: friss

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D80789