ARM: suuport .tlsdescseq directive

This enhances the ARMAsmParser to handle .tlsdescseq directives.  This is a
slightly special relocation.  We must be able to generate them, but not consume
them in assembly.  The relocation is meant to assist the linker in generating a
TLS descriptor sequence.  The ELF target streamer is enhanced to append
additional fixups into the current segment and that is used to emit the new
R_ARM_TLS_DESCSEQ relocations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200448 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCExpr.h

@@ -168,6 +168,7 @@ public:
     VK_ARM_TLSLDO,         // symbol(tlsldo)
     VK_ARM_TLSCALL,        // symbol(tlscall)
     VK_ARM_TLSDESC,        // symbol(tlsdesc)
+    VK_ARM_TLSDESCSEQ,
 
     VK_PPC_LO,             // symbol@l
     VK_PPC_HI,             // symbol@h

include/llvm/MC/MCStreamer.h

@@ -33,6 +33,7 @@ class MCInstPrinter;
 class MCSection;
 class MCStreamer;
 class MCSymbol;
+class MCSymbolRefExpr;
 class MCSubtargetInfo;
 class StringRef;
 class Twine;
@@ -107,6 +108,8 @@ public:
   virtual void emitArch(unsigned Arch) = 0;
   virtual void finishAttributeSection() = 0;
   virtual void emitInst(uint32_t Inst, char Suffix = '\0') = 0;
+
+  virtual void AnnotateTLSDescriptorSequence(const MCSymbolRefExpr *SRE) = 0;
 };
 
 /// MCStreamer - Streaming machine code generation interface.  This interface