[sancov] coverage-report-server.py: ServerHandler(): open file as UTF8

Summary:
This is nessesary in Python3. Everywhere else we assume that
encoding is UTF8. If we don't specify it here, the defaults
from the environment will be used, which may result in ASCII
decoder being used. And if the file is non-ASCII, then it
will crash:
```
  File "/usr/local/bin/coverage-report-server.py", line 168, in do_GET
    for line_no, line in enumerate(f, start=1)])
  File "/usr/local/bin/coverage-report-server.py", line 165, in <listcomp>
    ["<span class='{cls}'>{line}&nbsp;</span>".format(
  File "/usr/lib/python3.5/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 106: ordinal not in range(128)
```

Fixes https://bugs.llvm.org/show_bug.cgi?id=33548

Now, how would i add a testcase here?

Reviewers: m.ostapenko, kcc

Reviewed By: kcc

Subscribers: kcc, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D37661

llvm-svn: 313063