[Driver] Only use -lc++ on Fuchsia

[phosek]

Summary

The fact that libc++ depends on libc++abi and libunwind is an internal
detail that's captured by the libc++.so linker script.

cfe/trunk/lib/Driver/ToolChains/Fuchsia.cpp

void Fuchsia::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
   }
 }
 
 void Fuchsia::AddCXXStdlibLibArgs(const ArgList &Args,
                                   ArgStringList &CmdArgs) const {
   switch (GetCXXStdlibType(Args)) {
   case ToolChain::CST_Libcxx:
     CmdArgs.push_back("-lc++");
-    CmdArgs.push_back("-lc++abi");
-    CmdArgs.push_back("-lunwind");
     break;
 
   case ToolChain::CST_Libstdcxx:
     llvm_unreachable("invalid stdlib name");
   }
 }
 
 SanitizerMask Fuchsia::getSupportedSanitizers() const {

cfe/trunk/test/Driver/fuchsia.cpp

 // CHECK: "--sysroot=[[SYSROOT]]"
 // CHECK: "-pie"
 // CHECK: "--build-id"
 // CHECK: "-dynamic-linker" "ld.so.1"
 // CHECK: Scrt1.o
 // CHECK-NOT: crti.o
 // CHECK-NOT: crtbegin.o
 // CHECK: "-L[[SYSROOT]]{{/|\\\\}}lib"
-// CHECK: "-lc++" "-lc++abi" "-lunwind" "-lm"
+// CHECK: "-lc++" "-lm"
 // CHECK: "{{.*[/\\]}}libclang_rt.builtins-x86_64.a"
 // CHECK: "-lc"
 // CHECK-NOT: crtend.o
 // CHECK-NOT: crtn.o
 
 // RUN: %clangxx %s -### --target=x86_64-unknown-fuchsia -stdlib=libstdc++ 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-STDLIB
 // CHECK-STDLIB: error: invalid library name in argument '-stdlib=libstdc++'
 
 // RUN: %clangxx %s -### --target=x86_64-unknown-fuchsia -static-libstdc++ 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-STATIC
 // CHECK-STATIC-NOT: "-Bstatic"
-// CHECK-STATIC: "-lc++" "-lc++abi" "-lunwind"
+// CHECK-STATIC: "-lc++"
 // CHECK-STATIC-NOT: "-Bdynamic"
 // CHECK-STATIC: "-lm"
 // CHECK-STATIC: "-lc"