[analyzer] Fix conversion from LocAsInteger to Loc symbol.

[NoQ]

Summary

The thing i promised to fix in the last bullet of D44347:

SymbolReaper::markElementIndicesLive() scans an array symbol, which is always a NonLoc. However, it may still be a nonloc::LocAsInteger, and in this case descend to the symbolic base would be necessary, as demonstrated by test_loc_as_integer_element_index_lifetime(). This test, however, is not fixed by the current patch, due to getAsLocSymbol() incorrectly failing to pass its IncludeBaseRegions argument into the recursive call. But i decided not to investigate it further today, because, well, enough is enough.

lib/StaticAnalyzer/Core/SVals.cpp

 ///
 /// Implicit casts (ex: void* -> char*) can turn Symbolic region into Element
 /// region. If that is the case, gets the underlining region.
 /// When IncludeBaseRegions is set to true and the SubRegion is non-symbolic,
 /// the first symbolic parent region is returned.
 SymbolRef SVal::getAsLocSymbol(bool IncludeBaseRegions) const {
   // FIXME: should we consider SymbolRef wrapped in CodeTextRegion?
   if (Optional<nonloc::LocAsInteger> X = getAs<nonloc::LocAsInteger>())
-    return X->getLoc().getAsLocSymbol();
+    return X->getLoc().getAsLocSymbol(IncludeBaseRegions);
 
   if (Optional<loc::MemRegionVal> X = getAs<loc::MemRegionVal>()) {
     const MemRegion *R = X->getRegion();
     if (const SymbolicRegion *SymR = IncludeBaseRegions ?
                                       R->getSymbolicBase() :
                                       dyn_cast<SymbolicRegion>(R->StripCasts()))
       return SymR->getSymbol();
   }

test/Analysis/symbol-reaper.c

 void test_loc_as_integer_element_index_lifetime() {
   do {
     int x;
     struct S3 *s = conjure_S3();
     clang_analyzer_warnOnDeadSymbol((int)s);
     x = (int)&(s->field);
     ptr = &arr[x];
     if (s) {}
-  // FIXME: Should not warn. The symbol is still alive within the ptr's index.
-  } while (0); // expected-warning{{SYMBOL DEAD}}
+  } while (0);
 }
 
 // Test below checks lifetime of SymbolRegionValue in certain conditions.
 
 int **ptrptr;
 void test_region_lifetime_as_store_value(int *x) {
   clang_analyzer_warnOnDeadSymbol((int) x);
   *x = 1;