Make the big switch: Change MCSectionMachO to represent a section *semantically*

instead of syntactically as a string.  This means that it keeps track of the 
segment, section, flags, etc directly and asmprints them in the right format.
This also includes parsing and validation support for llvm-mc and 
"attribute(section)", so we should now start getting errors about invalid 
section attributes from the compiler instead of the assembler on darwin.

Still todo: 
1) Uniquing of darwin mcsections
2) Move all the Darwin stuff out to MCSectionMachO.[cpp|h]
3) there are a few FIXMEs, for example what is the syntax to get the
   S_GB_ZEROFILL segment type?



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78547 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-mc/llvm-mc.cpp

@@ -186,11 +186,10 @@ static int AssembleInput(const char *ProgName) {
   OwningPtr<MCStreamer> Str(createAsmStreamer(Ctx, outs()));
 
   // FIXME: Target hook & command line option for initial section.
-  Str.get()->SwitchSection(MCSectionMachO::Create("__TEXT,__text,"
-                                             "regular,pure_instructions",
-                                             false,
-                                            SectionKind::getText(),
-                                             Ctx));
+  Str.get()->SwitchSection(MCSectionMachO::Create("__TEXT","__text",
+                                       MCSectionMachO::S_ATTR_PURE_INSTRUCTIONS,
+                                                  0, SectionKind::getText(),
+                                                  Ctx));
 
   AsmParser Parser(SrcMgr, Ctx, *Str.get());
   OwningPtr<TargetAsmParser> TAP(GetTargetAsmParser(ProgName, Parser));