[opt-viewer] Handle file names that contain '#'

Summary:
When using opt-viewer.py with files with '#' in their name, such as
'foo#bar.cpp', opt-viewer.py would generate links such as
'/path/to/foo#bar.cpp.opt.yaml#L42'. In this case, the link is
interpreted by browsers as a link to the file '/path/to/foo', and to the
section within that file with ID 'bar.cpp.opt.yaml#L42'.

To work around this issue, replace '#' with '_' in file names and links
in opt-viewer.py.

Reviewers: anemet, davidxl

Reviewed By: davidxl

Subscribers: llvm-commits, fhahn

Differential Revision: https://reviews.llvm.org/D34646

llvm-svn: 308346