[clang] Make make_cxx_dr_status script runnable from anywhere

This script has hardcoded relative paths to `clang/test/CXX/drs`, `cwg_index.html`, and `cxx_dr_status.html`, which requires running it with `clang/www` CWD. This patch makes those paths relative to path of the script itself, so that it could be run from anywhere.

Reviewed By: #clang-language-wg, cor3ntin

Differential Revision: https://reviews.llvm.org/D148146

clang/www/make_cxx_dr_status

@@ -2,10 +2,11 @@
 import sys, os, re, urllib.request
 
 
-default_issue_list_path = 'cwg_index.html'
+clang_www_dir = os.path.dirname(__file__)
+default_issue_list_path = os.path.join(clang_www_dir, 'cwg_index.html')
 issue_list_url = "https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_index.html"
-output = 'cxx_dr_status.html'
-dr_test_dir = '../test/CXX/drs'
+output = os.path.join(clang_www_dir, 'cxx_dr_status.html')
+dr_test_dir = os.path.join(clang_www_dir, '../test/CXX/drs')
 
 class DR:
   def __init__(self, section, issue, url, status, title):