lit-tests: Forward env variables VSINSTALLDIR, PATH and LIB on Windows

In order not to fallback to the MinGW libs, which are most likely not
available at testing time. Use the MSVC toolchain used for building
instead, by forwarding the minimum set of required variables.

This also greatly reduces the overall runtime of these tests, as the VS
installation isn't auto-detected (incl. env variables setup) for every
single linking operation.

Author: kinke

tests/lit.site.cfg.in

@@ -63,6 +63,11 @@ env_cxx = os.environ.get('CXX', '')
 if env_cxx:
     config.environment['CXX'] = env_cxx
 
+if (platform.system() == 'Windows'):
+    config.environment['VSINSTALLDIR'] = os.environ['VSINSTALLDIR']
+    config.environment['PATH'] = os.environ['PATH']
+    config.environment['LIB'] = os.environ['LIB']
+
 # Define available features so that we can disable tests depending on LLVM version
 config.available_features.add("llvm%d" % config.llvm_version)
 # LLVM version history: 3.9, 4.0, 5.0, ...