[BOLT] Check if LLVM_REVISION is defined

[Amir]

Summary

Handle the case where LLVM_REVISION is undefined (due to LLVM_APPEND_VC_REV=OFF
or otherwise) by setting "<unknown>" value as before D123549.

bolt/lib/Utils/CommandLineOpts.cpp

 
 #include "bolt/Utils/CommandLineOpts.h"
 #include "llvm/Support/VCSRevision.h"
 
 using namespace llvm;
 
 namespace llvm {
 namespace bolt {
-const char *BoltRevision = LLVM_REVISION;
+const char *BoltRevision =
+#ifdef LLVM_REVISION
+    LLVM_REVISION;
+#else
+    "<unknown>";
+#endif
 }
 }
 
 namespace opts {
 
 bool HeatmapMode = false;
 bool LinuxKernelMode = false;