[Analyzer] [Tests] Fixing typo from the previous commit.

Can not open a non-existent file with r+.

llvm-svn: 316808
diff --git a/clang/utils/analyzer/SATestBuild.py b/clang/utils/analyzer/SATestBuild.py
index ed700c0..3c694fb 100755
--- a/clang/utils/analyzer/SATestBuild.py
+++ b/clang/utils/analyzer/SATestBuild.py
@@ -374,7 +374,7 @@
     os.makedirs(os.path.join(SBOutputDir, LogFolderName))
 
     # Build and analyze the project.
-    with open(BuildLogPath, "r+b") as PBuildLogFile:
+    with open(BuildLogPath, "wb+") as PBuildLogFile:
         if (ProjectBuildMode == 1):
             downloadAndPatch(Dir, PBuildLogFile)
             runCleanupScript(Dir, PBuildLogFile)