Use clang driver for libfuzzer tests on Windows

[rnk]

Summary

There's no real reason to use clang-cl on Windows, the clang driver
works just as well. This fixes a test which uses the -O0 flag, which was
recently removed from clang-cl to match MSVC, which lacks this flag.

While I'm here, remove the explicit -std=c++11 flag. Previously, this
flag was necessary when the default C++ standard was C++98. Now that the
default is C++14, this is no longer necessary. It's problematic on
Windows, because the Visual C++ standard library relies on C++14
features, and attempting to compile it with C++11 results in errors.
Rather than adding logic to conditionally set the standard to C++11 only
on non-Win, this flag can be removed.

See http://lab.llvm.org:8011/builders/clang-x64-windows-msvc and
https://reviews.llvm.org/D64506.

[morehouse]

LGTM

compiler-rt/trunk/test/fuzzer/lit.cfg.py

 config.substitutions.append(('%build_dir', config.cmake_binary_dir))
 libfuzzer_src_root = os.path.join(config.compiler_rt_src_root, "lib", "fuzzer")
 config.substitutions.append(('%libfuzzer_src', libfuzzer_src_root))
 
 def generate_compiler_cmd(is_cpp=True, fuzzer_enabled=True, msan_enabled=False):
   compiler_cmd = config.clang
   extra_cmd = config.target_flags
 
-  if is_cpp and 'windows-msvc' in config.target_triple:
-    std_cmd = '--driver-mode=cl'
-  elif is_cpp:
-    std_cmd = '--driver-mode=g++ -std=c++11'
+  if is_cpp:
+    std_cmd = '--driver-mode=g++'
   else:
     std_cmd = ''
 
   if msan_enabled:
     sanitizers = ['memory']
   else:
     sanitizers = ['address']
   if fuzzer_enabled:

compiler-rt/trunk/test/fuzzer/windows-opt-ref.test

 REQUIRES: windows
 // Verify that the linker eliminating unreferenced functions (/OPT:REF) does not
 // strip sancov module constructor.
-RUN: %cpp_compiler %S/SimpleCmpTest.cpp -o %t-SimpleCmpTest /link /OPT:REF
+RUN: %cpp_compiler %S/SimpleCmpTest.cpp -o %t-SimpleCmpTest -Wl,-OPT:REF
 
 RUN: not %run %t-SimpleCmpTest -seed=1 -runs=100000000 2>&1 | FileCheck %s
 
 CHECK-NOT: ERROR: no interesting inputs were found. Is the code instrumented for coverage? Exiting.
 CHECK: BINGO