Use 'env' in tests that set environment variables.

Patch by David Fang!

llvm-svn: 181861
diff --git a/clang/test/Tooling/auto-detect-from-source-parent-of-cwd.cpp b/clang/test/Tooling/auto-detect-from-source-parent-of-cwd.cpp
index 6b632b0..b1778a6 100644
--- a/clang/test/Tooling/auto-detect-from-source-parent-of-cwd.cpp
+++ b/clang/test/Tooling/auto-detect-from-source-parent-of-cwd.cpp
@@ -2,7 +2,7 @@
 // RUN: mkdir -p %t/abc/def/ijk/qwe
 // RUN: echo "[{\"directory\":\".\",\"command\":\"clang++ -c %t/abc/def/ijk/qwe/test.cpp\",\"file\":\"%t/abc/def/ijk/qwe/test.cpp\"}]" | sed -e 's/\\/\\\\/g' > %t/compile_commands.json
 // RUN: cp "%s" "%t/abc/def/ijk/qwe/test.cpp"
-// RUN: PWD="%t/abc/def" clang-check "ijk/qwe/test.cpp" 2>&1 | FileCheck %s
+// RUN: env PWD="%t/abc/def" clang-check "ijk/qwe/test.cpp" 2>&1 | FileCheck %s
 
 // CHECK: C++ requires
 invalid;