Fix clang installed path to handle case where clang is invoked through a symlink

This code path is used when generating the path to libLTO.dylib, which
is passed to the linker as `-lto_library'.
Without this, if clang is invoked through a symlink, libLTO is
searched in a path relative to where the symlink is instead of
where clang is actually installed.

Fix PR30811.

Patch by: Jack Howarth

Differential Revision: https://reviews.llvm.org/D26116

llvm-svn: 285525