[AIX][Debug] generate an error instead of crash in backend for -gdwarf-5

Before this change -gdwarf-5 on AIX will cause backend crash, because
some DWARF5 sections are not defined in XCOFF.
Explicitly statement -gdwarf-5 as unsupported in frontend on AIX.

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -2116,8 +2116,12 @@ unsigned tools::getDwarfVersion(const ToolChain &TC,
                                 const llvm::opt::ArgList &Args) {
   unsigned DwarfVersion = ParseDebugDefaultVersion(TC, Args);
   if (const Arg *GDwarfN = getDwarfNArg(Args))
-    if (int N = DwarfVersionNum(GDwarfN->getSpelling()))
+    if (int N = DwarfVersionNum(GDwarfN->getSpelling())) {
       DwarfVersion = N;
+      if (DwarfVersion == 5 && TC.getTriple().isOSAIX())
+        TC.getDriver().Diag(diag::err_drv_unsupported_opt_for_target)
+            << GDwarfN->getSpelling() << TC.getTriple().str();
+    }
   if (DwarfVersion == 0) {
     DwarfVersion = TC.GetDefaultDwarfVersion();
     assert(DwarfVersion && "toolchain default DWARF version must be nonzero");

clang/test/CodeGen/dwarf-version.c

@@ -46,8 +46,10 @@
 // RUN:   FileCheck %s --check-prefix=VER3
 // RUN: %clang -target powerpc-ibm-aix-xcoff -gdwarf-4 -S -emit-llvm -o - %s | \
 // RUN:   FileCheck %s --check-prefix=VER4
-// RUN: %clang -target powerpc-ibm-aix-xcoff -gdwarf-5 -S -emit-llvm -o - %s | \
-// RUN:   FileCheck %s --check-prefix=VER5
+// RUN: not %clang -target powerpc-ibm-aix-xcoff -gdwarf-5 -S -emit-llvm -o - %s 2>&1 | \
+// RUN:   FileCheck %s --check-prefix=UNSUPPORTED-VER5
+// RUN: not %clang -target powerpc64-ibm-aix-xcoff -gdwarf-5 -S -emit-llvm -o - %s 2>&1| \
+// RUN:   FileCheck %s --check-prefix=UNSUPPORTED-VER5
 
 int main (void) {
   return 0;
@@ -59,6 +61,7 @@ int main (void) {
 // VER3: !{i32 7, !"Dwarf Version", i32 3}
 // VER4: !{i32 7, !"Dwarf Version", i32 4}
 // VER5: !{i32 7, !"Dwarf Version", i32 5}
+// UNSUPPORTED-VER5: error: unsupported option '-gdwarf-5'
 
 // NODWARF-NOT: !"Dwarf Version"
 // CODEVIEW: !{i32 2, !"CodeView", i32 1}