[LiveDebugValues] Temporarily initialize MLocTracker::CurBB

Looks like code assumes that it will be always set, but it's not true:
https://reviews.llvm.org/D150420. This is temporarily suppression to enabled
stricter msan on a bot.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.h

@@ -656,7 +656,7 @@ class MLocTracker {
 
   // If we discover a new machine location, assign it an mphi with this
   // block number.
-  unsigned CurBB;
+  unsigned CurBB = 0;  // FIXME: https://reviews.llvm.org/D150420
 
   /// Cached local copy of the number of registers the target has.
   unsigned NumRegs;