[PATCH 1/2] [Driver] Make Driver::DefaultTargetTriple private (NFCI)

[vsk]

Summary

The clang driver knows about two kinds of target triples: default and
effective. Default triples are needed to get ToolChains. ToolChains can
then be used to determine more specific effective triples.

Inconsistent use of two different kinds of target triples leads to
inconsistent error reporting and wasted computation (n.b that effective
triples are not cached).

Ideally, default triples would only be used as a means of getting
effective triples. Make Driver::DefaultTargetTriple private in the hopes
of discouraging further use of default triples.

[dexonsmith]

If no one is using it, making it private is obvious goodness. LGTM.

include/clang/Driver/Driver.h

   /// If the standard library is used
   bool UseStdLib;
 
-  /// Default target triple.
-  std::string DefaultTargetTriple;
-
   /// Driver title to use with help.
   std::string DriverTitle;
 

   unsigned CCGenDiagnostics : 1;
 
 private:
+  /// Default target triple.
+  std::string DefaultTargetTriple;
+
   /// Name to use when invoking gcc/g++.
   std::string CCCGenericGCCName;
 

lib/Driver/Driver.cpp

       Mode(GCCMode), SaveTemps(SaveTempsNone), BitcodeEmbed(EmbedNone),
       LTOMode(LTOK_None), ClangExecutable(ClangExecutable),
       SysRoot(DEFAULT_SYSROOT), UseStdLib(true),
-      DefaultTargetTriple(DefaultTargetTriple),
       DriverTitle("clang LLVM compiler"), CCPrintOptionsFilename(nullptr),
       CCPrintHeadersFilename(nullptr), CCLogDiagnosticsFilename(nullptr),
       CCCPrintBindings(false), CCPrintHeaders(false), CCLogDiagnostics(false),
-      CCGenDiagnostics(false), CCCGenericGCCName(""), CheckInputsExist(true),
-      CCCUsePCH(true), SuppressMissingInputWarning(false) {
+      CCGenDiagnostics(false), DefaultTargetTriple(DefaultTargetTriple),
+      CCCGenericGCCName(""), CheckInputsExist(true), CCCUsePCH(true),
+      SuppressMissingInputWarning(false) {
 
   // Provide a sane fallback if no VFS is specified.
   if (!this->VFS)