[Driver][Fuchsia] Support -nolibc flag

[phosek]

Summary

This can be used to disable libc linking. This flag is supported by
GCC since version 9 as well as some Clang target toolchains. This
change also includes tests for all -no* flags which previously weren't
covered.

[mcgrathr]

This is only on GCC trunk (i.e. 9), not in 8.2 or even the current gcc-8 branch. So clarify the log entry. We don't know if 8.x,x>2 will add it or only gcc 9.

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

if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
 
     if (Args.hasArg(options::OPT_pthread) ||
         Args.hasArg(options::OPT_pthreads))
       CmdArgs.push_back("-lpthread");
 
     if (Args.hasArg(options::OPT_fsplit_stack))
       CmdArgs.push_back("--wrap=pthread_create");
 
+    if (!Args.hasArg(options::OPT_nolibc))
       CmdArgs.push_back("-lc");
   }
 
   C.addCommand(llvm::make_unique<Command>(JA, *this, Exec, CmdArgs, Inputs));
 }
 
 /// Fuchsia - Fuchsia tool chain which can call as(1) and ld(1) directly.
 
 Fuchsia::Fuchsia(const Driver &D, const llvm::Triple &Triple,

cfe/trunk/test/Driver/fuchsia.c

 // CHECK-SHARED: "-shared"
 
 // RUN: %clang %s -### --target=x86_64-fuchsia -r -fuse-ld=lld 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-RELOCATABLE
 // CHECK-RELOCATABLE-NOT: "-pie"
 // CHECK-RELOCATABLE-NOT: "--build-id"
 // CHECK-RELOCATABLE: "-r"
 
+// RUN: %clang %s -### --target=x86_64-fuchsia -nodefaultlibs -fuse-ld=lld 2>&1 \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     | FileCheck %s -check-prefix=CHECK-NODEFAULTLIBS
+// CHECK-NODEFAULTLIBS: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
+// CHECK-NODEFAULTLIBS-NOT: "[[RESOURCE_DIR]]{{/|\\\\}}x86_64-fuchsia{{/|\\\\}}lib{{/|\\\\}}libclang_rt.builtins.a"
+// CHECK-NODEFAULTLIBS-NOT: "-lc"
+
+// RUN: %clang %s -### --target=x86_64-fuchsia -nostdlib -fuse-ld=lld 2>&1 \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     | FileCheck %s -check-prefix=CHECK-NOSTDLIB
+// CHECK-NOSTDLIB: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
+// CHECK-NOSTDLIB-NOT: "[[RESOURCE_DIR]]{{/|\\\\}}x86_64-fuchsia{{/|\\\\}}lib{{/|\\\\}}libclang_rt.builtins.a"
+// CHECK-NOSTDLIB-NOT: "-lc"
+
+// RUN: %clang %s -### --target=x86_64-fuchsia -nolibc -fuse-ld=lld 2>&1 \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     | FileCheck %s -check-prefix=CHECK-NOLIBC
+// CHECK-NOLIBC: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
+// CHECK-NOLIBC: "[[RESOURCE_DIR]]{{/|\\\\}}x86_64-fuchsia{{/|\\\\}}lib{{/|\\\\}}libclang_rt.builtins.a"
+// CHECK-NOLIBC-NOT: "-lc"
+
 // RUN: %clang %s -### --target=x86_64-fuchsia \
 // RUN:     -fsanitize=safe-stack 2>&1 \
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
 // RUN:     -fuse-ld=lld \
 // RUN:     | FileCheck %s -check-prefix=CHECK-SAFESTACK
 // CHECK-SAFESTACK: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK-SAFESTACK: "-fsanitize=safe-stack"
 // CHECK-SAFESTACK-NOT: "[[RESOURCE_DIR]]{{/|\\\\}}x86_64-fuchsia{{/|\\\\}}lib{{/|\\\\}}libclang_rt.safestack.a"

cfe/trunk/test/Driver/fuchsia.cpp

 // CHECK-STATIC: "--push-state"
 // CHECK-STATIC: "--as-needed"
 // CHECK-STATIC: "-Bstatic"
 // CHECK-STATIC: "-lc++"
 // CHECK-STATIC: "-Bdynamic"
 // CHECK-STATIC: "-lm"
 // CHECK-STATIC: "--pop-state"
 // CHECK-STATIC: "-lc"
+
+// RUN: %clang %s -### --target=x86_64-fuchsia -nostdlib++ -fuse-ld=lld 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-NOSTDLIBXX
+// CHECK-NOSTDLIBXX-NOT: "-lc++"
+// CHECK-NOSTDLIBXX-NOT: "-lm"
+// CHECK-NOSTDLIBXX: "-lc"