[llc] Fix save-stats test in read only directories

The save stats test would fail downstream due to the tests being run
from a read only directory. The cwd flag would attempt to place the
statistics in that read only directory, causing an error. This patch
ensures that the tests are always run from a temp directory.

Author: michaelrj-google

llvm/test/tools/llc/save-stats.ll

@@ -1,5 +1,6 @@
 ; REQUIRES: asserts
 
+; RUN: rm -rf %t.dir && mkdir -p %t.dir && cd %t.dir
 ; RUN: llc --save-stats=obj -o %t.s %s && cat %t.stats | FileCheck %s
 ; RUN: llc --save-stats=cwd -o %t.s %s && cat %{t:stem}.tmp.stats | FileCheck %s
 ; RUN: llc --save-stats -o %t.s %s && cat %{t:stem}.tmp.stats | FileCheck %s