[Clang] Improve error for -fsanitize=function/kcfi -mexecute-only incompatibility

The current error message when using the `-fsanitize=function -mexecute-only`
flags together points to the target triple as the reason that
`-fsanitize=function` is not allowed to be used, even when the function
sanitizer is otherwise supported on the target when not using
`-mexecute-only`.

The error message is improved to give `-mexecute-only` as the reason for
disallowing `-fsanitize=function` if it was passed to the driver.

Author: Il-Capitano

clang/lib/Driver/SanitizerArgs.cpp

@@ -418,8 +418,14 @@ SanitizerArgs::SanitizerArgs(const ToolChain &TC,
                 Add & NotAllowedWithExecuteOnly & ~DiagnosedKinds) {
           if (DiagnoseErrors) {
             std::string Desc = describeSanitizeArg(Arg, KindsToDiagnose);
-            D.Diag(diag::err_drv_argument_not_allowed_with)
-                << Desc << Triple.str();
+            llvm::opt::Arg *A = Args.getLastArgNoClaim(
+                options::OPT_mexecute_only, options::OPT_mno_execute_only);
+            if (A && A->getOption().matches(options::OPT_mexecute_only))
+              D.Diag(diag::err_drv_argument_not_allowed_with)
+                  << Desc << A->getAsString(Args);
+            else
+              D.Diag(diag::err_drv_argument_not_allowed_with)
+                  << Desc << Triple.str();
           }
           DiagnosedKinds |= KindsToDiagnose;
         }

clang/test/Driver/fsanitize.c

@@ -1000,8 +1000,8 @@
 // RUN: not %clang --target=armv6t2-eabi -mexecute-only -fsanitize=kcfi %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UBSAN-KCFI
 // RUN: %clang --target=armv6t2-eabi -mexecute-only -fsanitize=undefined %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UBSAN-UNDEFINED-VPTR
 
-// CHECK-UBSAN-KCFI-DAG: error: invalid argument '-fsanitize=kcfi' not allowed with {{('x86_64-sie-ps5'|'armv6t2-unknown-unknown-eabi')}}
-// CHECK-UBSAN-FUNCTION-DAG: error: invalid argument '-fsanitize=function' not allowed with {{('x86_64-sie-ps5'|'armv6t2-unknown-unknown-eabi')}}
+// CHECK-UBSAN-KCFI-DAG: error: invalid argument '-fsanitize=kcfi' not allowed with {{('x86_64-sie-ps5'|'-mexecute-only')}}
+// CHECK-UBSAN-FUNCTION-DAG: error: invalid argument '-fsanitize=function' not allowed with {{('x86_64-sie-ps5'|'-mexecute-only')}}
 // CHECK-UBSAN-UNDEFINED-VPTR: "-fsanitize={{((alignment|array-bounds|bool|builtin|enum|float-cast-overflow|integer-divide-by-zero|nonnull-attribute|null|pointer-overflow|return|returns-nonnull-attribute|shift-base|shift-exponent|signed-integer-overflow|unreachable|vla-bound|vptr),?){18}"}}
 
 // * Test BareMetal toolchain sanitizer support *