Fix build failure when source is read only

cmake configure fails when it tries to setup target for llvm_vcsrevision_h
This happens only when source is checked out using repo in a read
only filesystem, because cmake tries to create `.git/logs/HEAD` file.

This patch:
  1. Recovers from failure gracefully.
  2. Ensures that VCSRevision.h is successfully created and updated
     in above scenarios.

Differential Revision: https://reviews.llvm.org/D79400