[llvm-dsymutil] Add support for __swift_ast MachO DWARF section

Summary:
Xcode's dsymutil emits a __swift_ast DWARF section, which is required for debugging,
and which contains a byte-for-byte dump of the swiftmodule file.
Add this feature to llvm-dsymutil.

Tested with `gobjdump --dwarf=info -s`, by verifying that the contents of
`__DWARF.__swift_ast` match between Xcode's dsymutil and llvm-dsymutil
(Xcode's dwarfdump and llvm-dwarfdump don't currently recognize the
__swift_ast section).

Reviewers: aprantl, friss

Subscribers: llvm-commits, JDevlieghere

Differential Revision: https://reviews.llvm.org/D38504

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315066 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCObjectFileInfo.cpp

@@ -214,6 +214,10 @@ void MCObjectFileInfo::initMachOMCObjectFileInfo(const Triple &T) {
       Ctx->getMachOSection("__DWARF", "__apple_types", MachO::S_ATTR_DEBUG,
                            SectionKind::getMetadata(), "types_begin");
 
+  DwarfSwiftASTSection =
+      Ctx->getMachOSection("__DWARF", "__swift_ast", MachO::S_ATTR_DEBUG,
+                           SectionKind::getMetadata());
+
   DwarfAbbrevSection =
       Ctx->getMachOSection("__DWARF", "__debug_abbrev", MachO::S_ATTR_DEBUG,
                            SectionKind::getMetadata(), "section_abbrev");