[clang-doc] Fix use of source-root flag

The value, if any, of --source-root flag was not being used.
This has been fixed and the logic was moved to the ClangDocContext
contructor.

Differential revision: https://reviews.llvm.org/D66268

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@369065 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed