[sancov] fix coverage-report-server cannot display coverage detail

This patch make following change for coverage-report-server.py
- using uri `./{name}` from root in the old version python http.server can be handled as `//{name}`. But due to https://github.com/python/cpython/pull/93879, it will be handled as `/{name}` now.

So I want to use a prefix to avoid double slashes issue.

Differential Revision: https://reviews.llvm.org/D146010

llvm/tools/sancov/coverage-report-server.py

@@ -71,6 +71,8 @@ $content
 </html>
 """
 
+FILE_URI_PREFIX = "/file/"
+
 class SymcovData:
     def __init__(self, symcov_json):
         self.covered_points = frozenset(symcov_json['covered-points'])
@@ -129,7 +131,7 @@ class ServerHandler(http.server.BaseHTTPRequestHandler):
     src_path = None
 
     def do_GET(self):
-        norm_path = os.path.normpath(urllib.parse.unquote(self.path[1:]))
+        norm_path = os.path.normpath(urllib.parse.unquote(self.path[len(FILE_URI_PREFIX):]))
         if self.path == '/':
             self.send_response(200)
             self.send_header("Content-type", "text/html; charset=utf-8")
@@ -141,8 +143,9 @@ class ServerHandler(http.server.BaseHTTPRequestHandler):
                 if not file_coverage:
                     continue
                 filelist.append(
-                        "<tr><td><a href=\"./{name}\">{name}</a></td>"
+                        "<tr><td><a href=\"{prefix}{name}\">{name}</a></td>"
                         "<td>{coverage}%</td></tr>".format(
+                            prefix=FILE_URI_PREFIX,
                             name=html.escape(filename, quote=True), 
                             coverage=format_pct(file_coverage)))