Fix executable names in lit.cfg

On Windows `%flang1` and `%flang2` are interpreted incorrectly.
related issue: flang-compiler#1363

test/lit.cfg

@@ -263,9 +263,11 @@ config.substitutions.append( ('%test_debuginfo', ' ' + config.llvm_src_root + '/
 config.substitutions.append( ('%itanium_abi_triple', makeItaniumABITriple(config.target_triple)) )
 config.substitutions.append( ('%ms_abi_triple', makeMSABITriple(config.target_triple)) )
 
-config.substitutions.append( ('%flang1', ' ' + config.flang + '1 ') )
-config.substitutions.append( ('%flang2', ' ' + config.flang + '2 ') )
-config.substitutions.append( ('%flang', ' ' + config.flang + ' ') )
+flang_root = os.path.splitext(config.flang)[0]
+flang_suffix = os.path.splitext(config.flang)[1]
+config.substitutions.append( ('%flang1', ' {}1{} '.format(flang_root, flang_suffix)) )
+config.substitutions.append( ('%flang2', ' {}2{} '.format(flang_root, flang_suffix)) )
+config.substitutions.append( ('%flang', ' {} '.format(config.flang)) )
 
 # The host triple might not be set, at least if we're compiling flang from
 # an already installed llvm.