[clang] Enable errors for undefined TARGET_OS_ macros in Darwin driver

[zixuw]

Summary

Add clang option -Wundef-prefix=TARGET_OS_ and -Werror=undef-prefix
to Darwin driver.

[arphaman]

Please add a test-case as well.

[zixuw]

Add a test case.

[jrmuizel]

This seems to have broken compilation for me on 10.15.

The stdio.h from the 10.15 SDK there's the following usage:

#if TARGET_OS_EMBEDDED
#define __swift_unavailable_on(osx_msg, ios_msg) __swift_unavailable(ios_msg)
#else
#define __swift_unavailable_on(osx_msg, ios_msg) __swift_unavailable(osx_msg)
#endif

[jrmuizel]

Actually, this may just be a local configuration issue.

clang/lib/Driver/ToolChains/Darwin.cpp

Tool *MachO::buildAssembler() const {
   return new tools::darwin::Assembler(*this);
 }
 
 DarwinClang::DarwinClang(const Driver &D, const llvm::Triple &Triple,
                          const ArgList &Args)
     : Darwin(D, Triple, Args) {}
 
 void DarwinClang::addClangWarningOptions(ArgStringList &CC1Args) const {
+  // Always error about undefined 'TARGET_OS_*' macros.
+  CC1Args.push_back("-Wundef-prefix=TARGET_OS_");
+  CC1Args.push_back("-Werror=undef-prefix");
+
   // For modern targets, promote certain warnings to errors.
   if (isTargetWatchOSBased() || getTriple().isArch64Bit()) {
     // Always enable -Wdeprecated-objc-isa-usage and promote it
     // to an error.
     CC1Args.push_back("-Wdeprecated-objc-isa-usage");
     CC1Args.push_back("-Werror=deprecated-objc-isa-usage");
 
     // For iOS and watchOS, also error about implicit function declarations,