[clang][Driver] use DWARF4 for wasm

[dschuff]

Summary

Opt into the old default of DWARF4 for now.

[dschuff]

Also +cc @pfaffe and @azakai

I'd like to land this soon to fix our emscripten tests for now. But we should talk about what the default should be. I'm curious if @sunfish has any any opinions on DWARF for their use cases and AFAIK lldb/chrome devtools is (or is almost?) ready for DWARF5 so maybe we can go back to defaulting to dwarf5 once we figure out exactly what to do for symbolizing stack traces of optimized builds (since Binaryen doesn't support DWARF5 yet, and may or may not soon).

[MaskRay]

clang/test/Driver/debug-options.c

141

Keep just one blank line

[dschuff]

just one blank line

[dschuff]

fix diff

clang/lib/Driver/ToolChains/WebAssembly.h

private:
   bool UseObjCMixedDispatch() const override;
   bool isPICDefault() const override;
   bool isPIEDefault(const llvm::opt::ArgList &Args) const override;
   bool isPICDefaultForced() const override;
   bool IsIntegratedAssemblerDefault() const override;
   bool hasBlocksRuntime() const override;
   bool SupportsProfiling() const override;
   bool HasNativeLLVMSupport() const override;
+  unsigned GetDefaultDwarfVersion() const override { return 4; }
   void
   addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
                         llvm::opt::ArgStringList &CC1Args,
                         Action::OffloadKind DeviceOffloadKind) const override;
   RuntimeLibType GetDefaultRuntimeLibType() const override;
   CXXStdlibType GetCXXStdlibType(const llvm::opt::ArgList &Args) const override;
   void
   AddClangSystemIncludeArgs(const llvm::opt::ArgList &DriverArgs,

clang/test/Driver/debug-options.c

 // RUN:             | FileCheck -check-prefix=NOCI %s
 // RUN: %clang -### -c -g %s -target powerpc64-ibm-aix-xcoff 2>&1 \
 // RUN:             | FileCheck -check-prefix=NOCI %s
 // RUN: %clang -### -c -g %s -target powerpc-ibm-aix-xcoff -gcolumn-info 2>&1 \
 // RUN:             | FileCheck -check-prefix=CI %s
 // RUN: %clang -### -c -g %s -target powerpc64-ibm-aix-xcoff -gcolumn-info \
 // RUN:             2>&1 | FileCheck -check-prefix=CI %s
 
+// WebAssembly.
+// WebAssembly should default to DWARF4.
+// RUN: %clang -### -c -g %s -target wasm32 2>&1 \
+// RUN:             | FileCheck -check-prefix=G_DWARF4 %s
+// RUN: %clang -### -c -g %s -target wasm64 2>&1 \
+// RUN:             | FileCheck -check-prefix=G_DWARF4 %s
+
 // RUN: %clang -### -c -gdwarf-2 %s 2>&1 \
 // RUN:             | FileCheck -check-prefix=G_ONLY_DWARF2 %s
 //
 // RUN: not %clang -### -c -gfoo %s 2>&1 | FileCheck -check-prefix=G_ERR %s
 // RUN: %clang -### -c -g -g0 %s 2>&1 | FileCheck -check-prefix=G_NO %s
 // RUN: %clang -### -c -ggdb0 %s 2>&1 | FileCheck -check-prefix=G_NO %s
 // RUN: %clang -### -c -glldb -g0 %s 2>&1 | FileCheck -check-prefix=G_NO %s
 // RUN: %clang -### -c -glldb -g1 %s 2>&1 \