[MC][X86] Add TrapFillValue definition

[Amir]

Summary

Adds the parametrized TrapFillValue that is used to fill the executable
space with instructions that trap. Used by BOLT to overwrite functions.

[MaskRay]

Sorry for being late on reviews.

But it is not clear TrapFillValue needs to be built-in in MC. I worry such magic treatment of text sections vs non-text sections.
You can use the .fill directive in assembly

[Amir]

Dropping this change, reimplemented in BOLT

llvm/include/llvm/MC/MCAsmInfo.h

protected:
   /// emits ".align log2(N)", e.g. 3 to align to an 8 byte boundary.  Defaults
   /// to true.
   bool AlignmentIsInBytes = true;
 
   /// If non-zero, this is used to fill the executable space created as the
   /// result of a alignment directive.  Defaults to 0
   unsigned TextAlignFillValue = 0;
 
+  /// If non-zero, this is used to fill the executable space with instructions
+  /// that will trap. Defaults to 0
+  unsigned TrapFillValue = 0;
+
   //===--- Global Variable Emission Directives --------------------------===//
 
   /// This is the directive used to declare a global entity. Defaults to
   /// ".globl".
   const char *GlobalDirective;
 
   /// True if the expression
   ///   .long f - g
public:
   const char *getAsciiDirective() const { return AsciiDirective; }
   const char *getAscizDirective() const { return AscizDirective; }
   const char *getByteListDirective() const { return ByteListDirective; }
   AsmCharLiteralSyntax characterLiteralSyntax() const {
     return CharacterLiteralSyntax;
   }
   bool getAlignmentIsInBytes() const { return AlignmentIsInBytes; }
   unsigned getTextAlignFillValue() const { return TextAlignFillValue; }
+  unsigned getTrapFillValue() const { return TrapFillValue; }
   const char *getGlobalDirective() const { return GlobalDirective; }
 
   bool doesSetDirectiveSuppressReloc() const {
     return SetDirectiveSuppressesReloc;
   }
 
   bool hasAggressiveSymbolFolding() const { return HasAggressiveSymbolFolding; }
 

llvm/lib/Target/X86/MCTargetDesc/X86MCAsmInfo.cpp

X86MCAsmInfoDarwin::X86MCAsmInfoDarwin(const Triple &T) {
   bool is64Bit = T.getArch() == Triple::x86_64;
   if (is64Bit)
     CodePointerSize = CalleeSaveStackSlotSize = 8;
 
   AssemblerDialect = AsmWriterFlavor;
 
   TextAlignFillValue = 0x90;
 
+  TrapFillValue = 0xCC;
+
   if (!is64Bit)
     Data64bitsDirective = nullptr;       // we can't emit a 64-bit unit
 
   // Use ## as a comment string so that .s files generated by llvm can go
   // through the GCC preprocessor without causing an error.  This is needed
   // because "clang foo.s" runs the C preprocessor, which is usually reserved
   // for .S files on other systems.  Perhaps this is because the file system
   // wasn't always case preserving or something.
X86ELFMCAsmInfo::X86ELFMCAsmInfo(const Triple &T) {
 
   // OTOH, stack slot size is always 8 for x86-64, even with the x32 ABI.
   CalleeSaveStackSlotSize = is64Bit ? 8 : 4;
 
   AssemblerDialect = AsmWriterFlavor;
 
   TextAlignFillValue = 0x90;
 
+  TrapFillValue = 0xCC;
+
   // Debug Information
   SupportsDebugInformation = true;
 
   // Exceptions handling
   ExceptionsType = ExceptionHandling::DwarfCFI;
 }
 
 const MCExpr *
X86MCAsmInfoMicrosoft::X86MCAsmInfoMicrosoft(const Triple &Triple) {
   }
 
   ExceptionsType = ExceptionHandling::WinEH;
 
   AssemblerDialect = AsmWriterFlavor;
 
   TextAlignFillValue = 0x90;
 
+  TrapFillValue = 0xCC;
+
   AllowAtInName = true;
 }
 
 void X86MCAsmInfoMicrosoftMASM::anchor() { }
 
 X86MCAsmInfoMicrosoftMASM::X86MCAsmInfoMicrosoftMASM(const Triple &Triple)
     : X86MCAsmInfoMicrosoft(Triple) {
   DollarIsPC = true;
X86MCAsmInfoGNUCOFF::X86MCAsmInfoGNUCOFF(const Triple &Triple) {
   } else {
     ExceptionsType = ExceptionHandling::DwarfCFI;
   }
 
   AssemblerDialect = AsmWriterFlavor;
 
   TextAlignFillValue = 0x90;
 
+  TrapFillValue = 0xCC;
+
   AllowAtInName = true;
 }