[ELF] - Add classof() member for ARMExidxSentinelSection.

Or code uses constructions like isa<ARMExidxSentinelSection>:
https://github.com/llvm-mirror/lld/blob/master/ELF/Writer.cpp#L1428
https://github.com/llvm-mirror/lld/blob/master/ELF/SyntheticSections.cpp#L2944

That is confusing, because without ARMExidxSentinelSection::classof()
these lines are equal to isa<SyntheticSection> and the code does not really do
the same what it expected to. I found no good way to break it though, but it is not nice.

Patch adds ARMExidxSentinelSection::classof().

llvm-svn: 336813

lld/ELF/SyntheticSections.cpp

@@ -2946,6 +2946,10 @@ bool ARMExidxSentinelSection::empty() const {
   return true;
 }
 
+bool ARMExidxSentinelSection::classof(const SectionBase *D) {
+  return D->kind() == InputSectionBase::Synthetic && D->Type == SHT_ARM_EXIDX;
+}
+
 ThunkSection::ThunkSection(OutputSection *OS, uint64_t Off)
     : SyntheticSection(SHF_ALLOC | SHF_EXECINSTR, SHT_PROGBITS,
                        Config->Wordsize, ".text.thunk") {

lld/ELF/SyntheticSections.h

@@ -924,6 +924,8 @@ public:
   void writeTo(uint8_t *Buf) override;
   bool empty() const override;
 
+  static bool classof(const SectionBase *D);
+
   // The last section referenced by a regular .ARM.exidx section.
   // It is found and filled in Writer<ELFT>::resolveShfLinkOrder().
   // The sentinel points at the end of that section.