[CrashReproducer] Rewrite relative include paths

When -fmodules is on, the reproducer invocation currently leave paths
for include-like flags as is. If the path is relative, the reproducer
doesn't know how to access that file during reproduction time because
the VFS cannot reason about relative paths.

Expand relative paths to absolute ones when creating the reproducer
command line. This allows, for example, the reproducer to work for
crashes while building clang with modules; this wasn't possible before
because building clang requires using relative inc dir from within the
build directory.

rdar://problem/28655070

llvm-svn: 289174

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