[BOLT] Fix reading LSDA address for PIC code

Summary:
Fix a bug while reading LSDA address in PIC format. The base address was
wrong for PC-relative value. There's more work involved in making PIC
code with C++ exceptions work.

(cherry picked from FBD5538755)

bolt/Exceptions.cpp

@@ -64,7 +64,7 @@ namespace bolt {
 //
 // The best visual representation of the tables comprising LSDA and
 // relationships between them is illustrated at:
-//   http://mentorembedded.github.io/cxx-abi/exceptions.pdf
+//   https://github.com/itanium-cxx-abi/cxx-abi/blob/master/exceptions.pdf
 // Keep in mind that GCC implementation deviates slightly from that document.
 //
 // To summarize, there are 4 tables in LSDA: call site table, actions table,
@@ -145,7 +145,7 @@ void BinaryFunction::parseLSDA(ArrayRef<uint8_t> LSDASectionData,
   intptr_t MaxTypeIndexTableOffset = 0;
 
   // The actual type info table starts at the same location, but grows in
-  // different direction. Encoding is different too (TTypeEncoding).
+  // opposite direction. TTypeEncoding is used to encode stored values.
   auto TypeTableStart = reinterpret_cast<const uint32_t *>(Ptr + TTypeEnd);
 
   uint8_t       CallSiteEncoding = *Ptr++;

bolt/RewriteInstance.cpp

@@ -3577,7 +3577,6 @@ void RewriteInstance::rewriteFile() {
   // Update ELF book-keeping info.
   patchELFSectionHeaderTable();
 
-  // TODO: we should find a way to mark the binary as optimized by us.
   Out->keep();
 
   // If requested, open again the binary we just wrote to dump its EH Frame