COFF: ICF: Include contents of referenced sections in initial partitioning hash. NFCI.

Previously we were taking over 13 minutes to link Firefox's xul.dll
on ARM64; this reduces link time to around 18s on my machine.

The root cause of the problem was that all of the input .pdata sections
had the same unrelocated section data and therefore the same hash,
which made segregation quadratic in the number of .pdata sections. The
reason why we weren't observing this on other architectures was that
ARM has a different .pdata format. On non-ARM the format is (start
address, end address, .xdata), which caused the size of the function
to appear in the unrelocated section data where the end address field
is. However, the ARM format omits the end address field.

Fixes PR39667.

Differential Revision: https://reviews.llvm.org/D54809

llvm-svn: 347429