Re-apply r211399, "Generate native unwind info on Win64" with a fix to ignore SEH pseudo ops in X86 JIT emitter.

--
This patch enables LLVM to emit Win64-native unwind info rather than
DWARF CFI.  It handles all corner cases (I hope), including stack
realignment.

Because the unwind info is not flexible enough to describe stack frames
with a gap of unknown size in the middle, such as the one caused by
stack realignment, I modified register spilling code to place all spills
into the fixed frame slots, so that they can be accessed relative to the
frame pointer.

Patch by Vadim Chugunov!

Reviewed By: rnk

Differential Revision: http://reviews.llvm.org/D4081

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211691 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCObjectFileInfo.cpp

@@ -649,11 +649,16 @@ void MCObjectFileInfo::InitCOFFMCObjectFileInfo(Triple T) {
   // though it contains relocatable pointers.  In PIC mode, this is probably a
   // big runtime hit for C++ apps.  Either the contents of the LSDA need to be
   // 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());
+  assert(T.isOSWindows() && "Windows is the only supported COFF target");
+  if (T.getArch() == Triple::x86_64) {
+    // On Windows 64 with SEH, the LSDA is emitted into the .xdata section
+    LSDASection = 0;
+  } else {
+    LSDASection = Ctx->getCOFFSection(".gcc_except_table",
+                                      COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
+                                          COFF::IMAGE_SCN_MEM_READ,
+                                      SectionKind::getReadOnly());
+  }
 
   // Debug info.
   COFFDebugSymbolsSection =