Revert "[Driver] Reject -mcmodel=tiny on X86 (#125643)"

This reverts commit c53e583.

Author: jplehr

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -2938,7 +2938,8 @@ void tools::addMCModel(const Driver &D, const llvm::opt::ArgList &Args,
       Ok = CM == "small" || CM == "medium" ||
            (CM == "large" && Triple.isRISCV64());
     } else if (Triple.getArch() == llvm::Triple::x86_64) {
-      Ok = llvm::is_contained({"small", "kernel", "medium", "large"}, CM);
+      Ok = llvm::is_contained({"small", "kernel", "medium", "large", "tiny"},
+                              CM);
     } else if (Triple.isNVPTX() || Triple.isAMDGPU() || Triple.isSPIRV()) {
       // NVPTX/AMDGPU/SPIRV does not care about the code model and will accept
       // whatever works for the host.

clang/test/Driver/mcmodel.c

@@ -1,5 +1,5 @@
 // RUN: not %clang -### -c --target=i686 -mcmodel=medium %s 2>&1 | FileCheck --check-prefix=ERR-MEDIUM %s
-// RUN: not %clang --target=x86_64 -### -c -mcmodel=tiny %s 2>&1 | FileCheck --check-prefix=ERR-TINY %s
+// RUN: %clang --target=x86_64 -### -c -mcmodel=tiny %s 2>&1 | FileCheck --check-prefix=TINY %s
 // RUN: %clang --target=x86_64 -### -c -mcmodel=small %s 2>&1 | FileCheck --check-prefix=SMALL %s
 // RUN: %clang --target=x86_64 -### -S -mcmodel=kernel %s 2>&1 | FileCheck --check-prefix=KERNEL %s
 // RUN: %clang --target=x86_64 -### -c -mcmodel=medium %s 2>&1 | FileCheck --check-prefix=MEDIUM %s