Speculative fix for build failures due to consumeInteger.

A recent patch added support for consumeInteger() and made
getAsInteger delegate to this function.  A few buildbots are
failing as a result with an assertion failure.  On a hunch,
I tested what happens if I call getAsInteger() on an empty
string, and sure enough it crashes the same way that the
buildbots are crashing.

I confirmed that getAsInteger() on an empty string did not
crash before my patch, so I suspect this to be the cause.

I also added a unit test for the empty string.

llvm-svn: 282170
diff --git a/llvm/unittests/ADT/StringRefTest.cpp b/llvm/unittests/ADT/StringRefTest.cpp
index 4249f23..40ab4e0 100644
--- a/llvm/unittests/ADT/StringRefTest.cpp
+++ b/llvm/unittests/ADT/StringRefTest.cpp
@@ -571,7 +571,8 @@
 
 
 static const char* BadStrings[] = {
-    "18446744073709551617"  // value just over max
+    ""                      // empty string
+  , "18446744073709551617"  // value just over max
   , "123456789012345678901" // value way too large
   , "4t23v"                 // illegal decimal characters
   , "0x123W56"              // illegal hex characters