[mips] Enable IAS by default for Android 64-bit MIPS target (N64)

[petarj]

Summary

IAS is already used for MIPS64 in majority of Android projects.
Android MIPS64 uses N64 ABI. Set IAS as a default now.

[sdardis]

LGTM with a comment clarity tweak.

lib/Driver/ToolChains/Gnu.cpp

2322–2323

Enabled for Debian and Android mips64/mipsel, as they can precisely
identify the ABI in use (Debian) or only use N64 for MIPS64 (Android).
Other targets are unable to distinguish N32 from N64.

lib/Driver/ToolChains/Gnu.cpp

bool Generic_GCC::IsIntegratedAssemblerDefault() const {
   case llvm::Triple::ppc64:
   case llvm::Triple::ppc64le:
   case llvm::Triple::systemz:
   case llvm::Triple::mips:
   case llvm::Triple::mipsel:
     return true;
   case llvm::Triple::mips64:
   case llvm::Triple::mips64el:
-    // Enabled for Debian mips64/mips64el only. Other targets are unable to
-    // distinguish N32 from N64.
-    if (getTriple().getEnvironment() == llvm::Triple::GNUABI64)
+    // Enabled for Debian mips64/mips64el and Android (uses N64 ABI).
+    // Other targets are unable to distinguish N32 from N64.
+    if (getTriple().getEnvironment() == llvm::Triple::GNUABI64 ||
+        getTriple().isAndroid())
       return true;
     return false;
   default:
     return false;
   }
 }
 
 void Generic_GCC::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,