Driver: simplify the profiler linking on Windows
Rather than computing an absolute path relative to Swift's resource
directory, use the compiler driver to locate the profiling runtime
relative to the C/C++ compiler's resource directory. This ensures that
we correctly locate the runtime. Additionally, because clang adds the
clang resource directory to the library search path, we do not need to
compute the path and can rely on the linker locating the runtime via the
library search path. This simplifies the handling for the profile
runtime linking on Windows.
Out of abundant paranoia, place the library link request after the
forced symbol inclusion as a GC root to ensure that `/opt:ref` will not
accidentally dead strip the symbol and force a reload of the library.
