Make lit TestRunner.py work in Python 3

Summary: In Python3 SubstituteCaptures are no longer converted to String implicitly behind the scenes. Converting explicitly makes the TestRunner to work in Python3.

Reviewers: gribozavr2, compnerd

Reviewed By: gribozavr2

Subscribers: tbkka, delcypher, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81361

(cherry picked from commit b49d47f)

Author: hlopko

llvm/utils/lit/lit/TestRunner.py

@@ -1163,7 +1163,7 @@ def processLine(ln):
             # short-lived, since the set of substitutions is fairly small, and
             # since thrashing has such bad consequences, not bounding the cache
             # seems reasonable.
-            ln = _caching_re_compile(a).sub(b, ln)
+            ln = _caching_re_compile(a).sub(str(b), ln)
 
         # Strip the trailing newline and any extra whitespace.
         return ln.strip()