PDB: Mark extended file pages as free by default.

BitVector::extend initializes extended bits as true by default.
That is not desirable because new pages should be initially free.

Differential Revision: https://reviews.llvm.org/D23048

llvm-svn: 277529

llvm/lib/DebugInfo/MSF/MSFBuilder.cpp

@@ -53,7 +53,7 @@ Error MSFBuilder::setBlockMapAddr(uint32_t Addr) {
     if (!IsGrowable)
       return make_error<MSFError>(msf_error_code::insufficient_buffer,
                                   "Cannot grow the number of blocks");
-    FreeBlocks.resize(Addr + 1);
+    FreeBlocks.resize(Addr + 1, true);
   }
 
   if (!isBlockFree(Addr))