SourceManager: Simplify early returns in ContentCache::getBufferOrNone, NFC

[dexonsmith]

Summary

As suggested in the review for https://reviews.llvm.org/D89430, simplify
the logic for marking the buffer as invalid in the early return paths.

clang/lib/Basic/SourceManager.cpp

ContentCache::getBufferOrNone(DiagnosticsEngine &Diag, FileManager &FM,
   // computed it, just return what we have.
   if (IsBufferInvalid)
     return None;
   if (Buffer)
     return Buffer->getMemBufferRef();
   if (!ContentsEntry)
     return None;
 
+  // Start with the assumption that the buffer is invalid to simplify early
+  // return paths.
+  IsBufferInvalid = true;
+
   // Check that the file's size fits in an 'unsigned' (with room for a
   // past-the-end value). This is deeply regrettable, but various parts of
   // Clang (including elsewhere in this file!) use 'unsigned' to represent file
   // offsets, line numbers, string literal lengths, and so on, and fail
   // miserably on large source files.
   if ((uint64_t)ContentsEntry->getSize() >=
       std::numeric_limits<unsigned>::max()) {
     if (Diag.isDiagnosticInFlight())
       Diag.SetDelayedDiagnostic(diag::err_file_too_large,
                                 ContentsEntry->getName());
     else
       Diag.Report(Loc, diag::err_file_too_large)
         << ContentsEntry->getName();
 
-    IsBufferInvalid = true;
     return None;
   }
 
   auto BufferOrError = FM.getBufferForFile(ContentsEntry, IsFileVolatile);
 
   // If we were unable to open the file, then we are in an inconsistent
   // situation where the content cache referenced a file which no longer
   // exists. Most likely, we were using a stat cache with an invalid entry but
   // the file could also have been removed during processing. Since we can't
   // really deal with this situation, just create an empty buffer.
   if (!BufferOrError) {
     if (Diag.isDiagnosticInFlight())
       Diag.SetDelayedDiagnostic(diag::err_cannot_open_file,
                                 ContentsEntry->getName(),
                                 BufferOrError.getError().message());
     else
       Diag.Report(Loc, diag::err_cannot_open_file)
           << ContentsEntry->getName() << BufferOrError.getError().message();
 
-    IsBufferInvalid = true;
     return None;
   }
 
   Buffer = std::move(*BufferOrError);
 
   // Check that the file's size is the same as in the file entry (which may
   // have come from a stat cache).
   if (getRawBuffer()->getBufferSize() != (size_t)ContentsEntry->getSize()) {
     if (Diag.isDiagnosticInFlight())
       Diag.SetDelayedDiagnostic(diag::err_file_modified,
                                 ContentsEntry->getName());
     else
       Diag.Report(Loc, diag::err_file_modified)
         << ContentsEntry->getName();
 
-    IsBufferInvalid = true;
     return None;
   }
 
   // If the buffer is valid, check to see if it has a UTF Byte Order Mark
   // (BOM).  We only support UTF-8 with and without a BOM right now.  See
   // http://en.wikipedia.org/wiki/Byte_order_mark for more information.
   StringRef BufStr = Buffer->getBuffer();
   const char *InvalidBOM = getInvalidBOM(BufStr);
 
   if (InvalidBOM) {
     Diag.Report(Loc, diag::err_unsupported_bom)
       << InvalidBOM << ContentsEntry->getName();
-    IsBufferInvalid = true;
     return None;
   }
 
+  // Buffer has been validated.
+  IsBufferInvalid = false;
   return Buffer->getMemBufferRef();
 }
 
 unsigned LineTableInfo::getLineTableFilenameID(StringRef Name) {
   auto IterBool = FilenameIDs.try_emplace(Name, FilenamesByID.size());
   if (IterBool.second)
     FilenamesByID.push_back(&*IterBool.first);
   return IterBool.first->second;