Add a comment on partial hashing of Metadata

Following r263866, on D. Blaikie suggestion.

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@263869 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed