Correctly escape %.

Found be an MSVC warning; I filed PR30320 for adding a similar warning to clang.

llvm-svn: 280900
diff --git a/compiler-rt/lib/profile/InstrProfilingFile.c b/compiler-rt/lib/profile/InstrProfilingFile.c
index fab479d..2c1ca95 100644
--- a/compiler-rt/lib/profile/InstrProfilingFile.c
+++ b/compiler-rt/lib/profile/InstrProfilingFile.c
@@ -558,7 +558,7 @@
 int __llvm_profile_dump(void) {
   if (!doMerging())
     PROF_WARN("Later invocation of __llvm_profile_dump can lead to clobbering "
-              " of previously dumped profile data : %s. Either use \%m "
+              " of previously dumped profile data : %s. Either use %%m "
               "in profile name or change profile name before dumping.\n",
               "online profile merging is not on");
   int rc = __llvm_profile_write_file();