Use the same tls section name as msvc.

We currently error in clang with:
"error: thread-local storage is unsupported for the current target", but we
can start to get the llvm level ready.

When compiling

template<typename T>
struct foo {
  static __declspec(thread) int bar;
};
template<typename T>
__declspec(therad) int foo<T>::bar;
template struct foo<int>;

msvc produces

SECTION HEADER #3
   .tls$ name
       0 physical address
       0 virtual address
       4 size of raw data
     12F file pointer to raw data (0000012F to 00000132)
       0 file pointer to relocation table
       0 file pointer to line numbers
       0 number of relocations
       0 number of line numbers
C0301040 flags
         Initialized Data
         COMDAT; sym= "public: static int foo<int>::bar" (?bar@?$foo@H@@2HA)
         4 byte align
         Read Write

gcc produces a ".data$__emutls_v.<symbol>" for the testcase with
__declspec(thread) replaced with thread_local.

llvm-svn: 195849

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -744,7 +744,7 @@ static const char *getCOFFSectionNameForUniqueGlobal(SectionKind Kind) {
   if (Kind.isBSS ())
     return ".bss";
   if (Kind.isThreadLocal())
-    return ".tls";
+    return ".tls$";
   if (Kind.isWriteable())
     return ".data";
   return ".rdata";

test/MC/COFF/weak-symbol.ll

@@ -36,3 +36,13 @@ define weak void @f() section ".sect" {
 ; X64: .globl a
 ; X64: .zero 12
 @a = weak unnamed_addr constant { i32, i32, i32 } { i32 0, i32 0, i32 0}, section ".data"
+
+; X86:  .section        .tls$,"w",discard,_b
+; X86:  .globl  _b
+; X86:  .long   0
+;
+; X64:  .section        .tls$,"w",discard,b
+; X64:  .globl  b
+; X64:  .long   0
+
+@b = weak_odr thread_local global i32 0, align 4