[ARM] Forbid use of TLS with execute-only (#124806)

Thread-local code generation requires constant pools because most of the
relocations needed for it operate on data, so it cannot be used with
-mexecute-only (or -mpure-code, which is aliased in the driver).

Without this we hit an assertion in the backend when trying to generate
a constant pool.

Author: ostannard

clang/lib/Basic/Targets/ARM.cpp

@@ -608,6 +608,8 @@ bool ARMTargetInfo::handleTargetFeatures(std::vector<std::string> &Features,
       HasBTI = 1;
     } else if (Feature == "+fullbf16") {
       HasFullBFloat16 = true;
+    } else if (Feature == "+execute-only") {
+      TLSSupported = false;
     }
   }
 

clang/test/Sema/arm-execute-only-tls.c

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -triple arm-none-eabi                               -fsyntax-only -verify=default      %s
+// RUN: %clang_cc1 -triple arm-none-eabi -target-feature +execute-only -fsyntax-only -verify=execute-only %s
+
+// default-no-diagnostics
+
+/// Thread-local code generation requires constant pools because most of the
+/// relocations needed for it operate on data, so it cannot be used with
+/// -mexecute-only (or -mpure-code, which is aliased in the driver).
+
+_Thread_local int t; // execute-only-error {{thread-local storage is not supported for the current target}}