[clang] Add option for -nolibc in Driver/ToolChains/Baremetal.cpp (#145700)

Some tests in LLVM-libc require this flag
(#145349), which requires
compiler-rt to be linked in, but not the C library. With this change,
the `-nolibc` flag will not be ignored.

Author: saturn691

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -671,7 +671,8 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     CmdArgs.push_back("--start-group");
     AddRunTimeLibs(TC, D, CmdArgs, Args);
-    CmdArgs.push_back("-lc");
+    if (!Args.hasArg(options::OPT_nolibc))
+      CmdArgs.push_back("-lc");
     if (TC.hasValidGCCInstallation() || detectGCCToolchainAdjacent(D))
       CmdArgs.push_back("-lgloss");
     CmdArgs.push_back("--end-group");

clang/test/Driver/baremetal.cpp

@@ -163,6 +163,16 @@
 // RUN:   | FileCheck %s --check-prefix=CHECK-RTLIB-GCC
 // CHECK-RTLIB-GCC: -lgcc
 
+// RUN: %clang -### --target=arm-none-eabi -nolibc -rtlib=compiler-rt %s 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-NOLIBC
+// CHECK-NOLIBC-NOT: "-lc"
+// CHECK-NOLIBC: "{{[^"]*}}libclang_rt.builtins.a"
+
+// RUN: %clang -### --target=arm-none-eabi -nostdlib -rtlib=compiler-rt %s 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-NOSTDLIB
+// CHECK-NOSTDLIB-NOT: "-lc"
+// CHECK-NOSTDLIB-NOT: "{{[^"]*}}libclang_rt.builtins.a"
+
 // RUN: %clang -### --target=arm-none-eabi -v %s 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-SYSROOT-INC
 // CHECK-SYSROOT-INC-NOT: "-internal-isystem" "include"