[Profile] Include a few asserts in coverage mapping test

[phosek]

Summary

These should catch any accidental use of the compilation directory.

[vsk]

Thanks!

llvm/unittests/ProfileData/CoverageMappingTest.cpp

struct CoverageMappingTest : ::testing::TestWithParam<std::pair<bool, bool>> {
 
   void SetUp() override {
     ProfileWriter.setOutputSparse(GetParam().first);
     UseMultipleReaders = GetParam().second;
   }
 
   unsigned getGlobalFileIndex(StringRef Name) {
     auto R = Files.find(Name);
-    if (R != Files.end())
+    if (R != Files.end()) {
+      assert(R->second > 0 && "got index of compilation dir");
       return R->second;
+    }
     unsigned Index = Files.size() + 1;
     Files.try_emplace(Name, Index);
     return Index;
   }
 
   // Return the file index of file 'Name' for the current function.
   // Add the file into the global map if necessary.
   // See also InputFunctionCoverageData::ReverseVirtualFileMapping
   // for additional comments.
   unsigned getFileIndexForFunction(StringRef Name) {
     unsigned GlobalIndex = getGlobalFileIndex(Name);
     auto &CurrentFunctionFileMapping =
         InputFunctions.back().ReverseVirtualFileMapping;
     auto R = CurrentFunctionFileMapping.find(GlobalIndex);
-    if (R != CurrentFunctionFileMapping.end())
+    if (R != CurrentFunctionFileMapping.end()) {
+      assert(R->second > 0 && "got index of compilation dir");
       return R->second;
+    }
     unsigned IndexInFunction = CurrentFunctionFileMapping.size();
     CurrentFunctionFileMapping.insert(
         std::make_pair(GlobalIndex, IndexInFunction));
     return IndexInFunction;
   }
 
   void startFunction(StringRef FuncName, uint64_t Hash) {
     InputFunctions.emplace_back(FuncName.str(), Hash);