[PATCH] Generate SEH unwinding info on Win64

[Saleem Abdulrasool]

================
Comment at: lib/MC/MCObjectFileInfo.cpp:635
@@ -634,6 +634,3 @@
   // adjusted or this should be a data section.
-  LSDASection =
-    Ctx->getCOFFSection(".gcc_except_table",
-                        COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
-                        COFF::IMAGE_SCN_MEM_READ,
-                        SectionKind::getReadOnly());
+  if (T.isOSWindows() && T.getArch() == Triple::x86_64) {
+    // On Windows 64 with SEH, the LSDA is emitted into the .xdata section
----------------
AFAIK, the COFF backend only supports Windows ATM.  Why not make this an assert?

    assert(T.isOSWindows() && "Windows is the only supported COFF target");
    if (T.getArch() != Triple::x86_64) {
      LSDASection = ...
    }

================
Comment at: lib/Target/X86/MCTargetDesc/X86MCAsmInfo.cpp:168
@@ -164,1 +167,3 @@
 
+  if (Triple.isOSWindows() && Triple.getArch() == Triple::x86_64)
+    ExceptionsType = ExceptionHandling::Win64;
----------------
Convert to an assert and simplify?

http://reviews.llvm.org/D4081