[cxx-interop][test] cleanup the lit.local.cfg for reverse Interop tests

Author: hyp

test/Interop/lit.local.cfg

@@ -6,16 +6,19 @@ def get_target_os():
     (run_cpu, run_vendor, run_os, run_version) = re.match('([^-]+)-([^-]+)-([^0-9]+)(.*)', config.variant_triple).groups()
     return run_os
 
-libc_opt = ''
 clang_opt = ''
 
 if get_target_os() in ['windows-msvc']:
     config.substitutions.insert(0, ('%target-abi', 'WIN'))
-    #libc_opt = '-libc MT '
+    # Clang should build object files with link settings equivalent to -libc MD
+    # when building for the MSVC target.
     clang_opt = '-D_MT -D_DLL -Xclang --dependent-lib=msvcrt -Xclang --dependent-lib=oldnames '
 else:
     # FIXME(compnerd) do all the targets we currently support use SysV ABI?
     config.substitutions.insert(0, ('%target-abi', 'SYSV'))
 
-config.substitutions.insert(0, ('%target-interop-build-swift', '%target-build-swift -Xfrontend -enable-cxx-interop ' + libc_opt))
+# Enable C++ interop when compiling Swift sources.
+config.substitutions.insert(0, ('%target-interop-build-swift',
+                                '%target-build-swift -Xfrontend -enable-cxx-interop '))
+# Build C++ files with matching link settings, if required by the target.
 config.substitutions.insert(0, ('%target-interop-build-clangxx', '%target-clangxx ' + clang_opt))