[AddressSanitizer] Put shadow at 0 for Fuchsia

[mcgrathr]

Summary

The Fuchsia ASan runtime reserves the low part of the address space.

[kcc]

LGTM
Lovely!

[eugenis]

AFAIK, JITs and such like to place generated code at 32-bit addresses (i.e. MAP_32BIT). Can this be an issue for Fuchsia?

[mcgrathr]

wrt JITs and such, if this becomes an issue we may revisit.

lib/Transforms/Instrumentation/AddressSanitizer.cpp

static ShadowMapping getShadowMapping(Triple &TargetTriple, int LongSize,
   bool IsX86 = TargetTriple.getArch() == llvm::Triple::x86;
   bool IsX86_64 = TargetTriple.getArch() == llvm::Triple::x86_64;
   bool IsMIPS32 = TargetTriple.getArch() == llvm::Triple::mips ||
                   TargetTriple.getArch() == llvm::Triple::mipsel;
   bool IsMIPS64 = TargetTriple.getArch() == llvm::Triple::mips64 ||
                   TargetTriple.getArch() == llvm::Triple::mips64el;
   bool IsAArch64 = TargetTriple.getArch() == llvm::Triple::aarch64;
   bool IsWindows = TargetTriple.isOSWindows();
+  bool IsFuchsia = TargetTriple.isOSFuchsia();
 
   ShadowMapping Mapping;
 
   if (LongSize == 32) {
     // Android is always PIE, which means that the beginning of the address
     // space is always available.
     if (IsAndroid)
       Mapping.Offset = 0;
     else if (IsMIPS32)
       Mapping.Offset = kMIPS32_ShadowOffset32;
     else if (IsFreeBSD)
       Mapping.Offset = kFreeBSD_ShadowOffset32;
     else if (IsIOS)
       // If we're targeting iOS and x86, the binary is built for iOS simulator.
       Mapping.Offset = IsX86 ? kIOSSimShadowOffset32 : kIOSShadowOffset32;
     else if (IsWindows)
       Mapping.Offset = kWindowsShadowOffset32;
     else
       Mapping.Offset = kDefaultShadowOffset32;
   } else {  // LongSize == 64
-    if (IsPPC64)
+    // Fuchsia is always PIE, which means that the beginning of the address
+    // space is always available.
+    if (IsFuchsia)
+      Mapping.Offset = 0;
+    else if (IsPPC64)
       Mapping.Offset = kPPC64_ShadowOffset64;
     else if (IsSystemZ)
       Mapping.Offset = kSystemZ_ShadowOffset64;
     else if (IsFreeBSD)
       Mapping.Offset = kFreeBSD_ShadowOffset64;
     else if (IsPS4CPU)
       Mapping.Offset = kPS4CPU_ShadowOffset64;
     else if (IsLinux && IsX86_64) {