[Driver] Default to DWARF 5 for Fuchsia

[phosek]

Summary

We use DWARF 5 on Fuchsia, make it the toolchain default.

[mcgrathr]

We aren't actually using DWARF 5 yet AFAICT. Our builds don't pass -gdwarf-5. So I'm not sure we have yet verified that all the DWARF-consuming tools people are using with Fuchsia binaries can handle all of DWARF 5 (which has several major format changes). I'd certainly like 5 to be the default, but I think we need to establish a set of consumers we care about and verify their format version support before we can be sure about this.

[aprantl]

I should also point out that even in DWARF v5 mode LLVM does not yet emit DWARF 5 variants of all sections as DWARF v5 support in LLVM is not yet feature-complete.

[phosek]

@mcgrathr This might be ready to land now.

[mcgrathr]

lgtm, but we should be sure to advise users who may need to use -gdwarf-4 explicitly if their debugging tools become unhappy.

[phosek]

No longer needed, this is now the Clang default.

clang/lib/Driver/ToolChains/Fuchsia.h

CXXStdlibType GetDefaultCXXStdlibType() const override {
     return ToolChain::CST_Libcxx;
   }
   bool IsUnwindTablesDefault(const llvm::opt::ArgList &Args) const override {
     return true;
   }
   bool isPICDefault() const override { return false; }
   bool isPIEDefault() const override { return true; }
   bool isPICDefaultForced() const override { return false; }
+  unsigned GetDefaultDwarfVersion() const override { return 5; }
   llvm::DebuggerKind getDefaultDebuggerTuning() const override {
     return llvm::DebuggerKind::GDB;
   }
 
   unsigned GetDefaultStackProtectorLevel(bool KernelOrKext) const override {
     return 2; // SSPStrong
   }
 

clang/test/Driver/fuchsia.c

 // CHECK-SHARED: "-shared"
 
 // RUN: %clang %s -### --target=x86_64-unknown-fuchsia -r 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-RELOCATABLE
 // CHECK-RELOCATABLE-NOT: "-pie"
 // CHECK-RELOCATABLE-NOT: "--build-id"
 // CHECK-RELOCATABLE: "-r"
 
+// RUN: %clang %s -### --target=x86_64-unknown-fuchsia -g 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-DEBUG
+// CHECK-DEBUG: "-dwarf-version=5"
+
 // RUN: %clang %s -### --target=x86_64-unknown-fuchsia \
 // RUN:     -fsanitize=safe-stack 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-SAFESTACK
 // CHECK-SAFESTACK: "-fsanitize=safe-stack"
 // CHECK-SAFESTACK-NOT: "{{.*[/\\]}}libclang_rt.safestack-x86_64.a"
 // CHECK-SAFESTACK-NOT: "__safestack_init"
 
 // RUN: %clang %s -### --target=x86_64-unknown-fuchsia \