Do not modify string returned by getenv on Windows. 
Fixes PR9875, patch by Nikola Smiljanic!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135356 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/InitHeaderSearch.cpp b/lib/Frontend/InitHeaderSearch.cpp
index c19e80a..e11a415 100644
--- a/lib/Frontend/InitHeaderSearch.cpp
+++ b/lib/Frontend/InitHeaderSearch.cpp
@@ -420,10 +420,8 @@
     vscomntools = vs80comntools;
 
   if (vscomntools && *vscomntools) {
-    char *p = const_cast<char *>(strstr(vscomntools, "\\Common7\\Tools"));
-    if (p)
-      *p = '\0';
-    path = vscomntools;
+    const char *p = strstr(vscomntools, "\\Common7\\Tools");
+    path = p ? std::string(vscomntools, p) : vscomntools;
     return true;
   }
   return false;