[lit] Fix UnicodeEncodeError when test commands contain non-ASCII chars

Ensure that the bash script written by lit TestRunner is open with UTF-8
encoding when using Python 3.  Otherwise, attempt to write non-ASCII
characters causes UnicodeEncodeError.  This happened e.g. with
the following LLD test:

UNRESOLVED: lld :: ELF/format-binary-non-ascii.s (657 of 2119)
******************** TEST 'lld :: ELF/format-binary-non-ascii.s' FAILED ********************
Exception during script execution:
Traceback (most recent call last):
  File "/home/mgorny/llvm-project/llvm/utils/lit/lit/worker.py", line 63, in _execute_test
    result = test.config.test_format.execute(test, lit_config)
  File "/home/mgorny/llvm-project/llvm/utils/lit/lit/formats/shtest.py", line 25, in execute
    self.execute_external)
  File "/home/mgorny/llvm-project/llvm/utils/lit/lit/TestRunner.py", line 1644, in executeShTest
    res = _runShTest(test, litConfig, useExternalSh, script, tmpBase)
  File "/home/mgorny/llvm-project/llvm/utils/lit/lit/TestRunner.py", line 1590, in _runShTest
    res = executeScript(test, litConfig, tmpBase, script, execdir)
  File "/home/mgorny/llvm-project/llvm/utils/lit/lit/TestRunner.py", line 1157, in executeScript
    f.write('{ ' + '; } &&\n{ '.join(commands) + '; }')
UnicodeEncodeError: 'ascii' codec can't encode character '\xa3' in position 274: ordinal not in range(128)

Differential Revision: https://reviews.llvm.org/D63254

llvm-svn: 363388