Revert "[Driver] Default riscv*- triples to -fdebug-default-version=4" (#84119)

This reverts commit bbc0f99
(https://reviews.llvm.org/D157663).

With this change, `-g` for the next major release 19.1 will generate
R_RISCV_SET_ULEB128/R_RISCV_SUB_ULEB128 relocations, which require
lld>=18 or binutils>=2.41.
binutils 2.41 is relatively new, but GCC has been producing
R_RISCV_SET_ULEB128/R_RISCV_SUB_ULEB128 for some time now.

Author: MaskRay

clang/include/clang/Driver/ToolChain.h

@@ -580,7 +580,7 @@ class ToolChain {
 
   // Return the DWARF version to emit, in the absence of arguments
   // to the contrary.
-  virtual unsigned GetDefaultDwarfVersion() const;
+  virtual unsigned GetDefaultDwarfVersion() const { return 5; }
 
   // Some toolchains may have different restrictions on the DWARF version and
   // may need to adjust it. E.g. NVPTX may need to enforce DWARF2 even when host

clang/lib/Driver/ToolChain.cpp

@@ -453,12 +453,6 @@ ToolChain::getDefaultUnwindTableLevel(const ArgList &Args) const {
   return UnwindTableLevel::None;
 }
 
-unsigned ToolChain::GetDefaultDwarfVersion() const {
-  // TODO: Remove the RISC-V special case when R_RISCV_SET_ULEB128 linker
-  // support becomes more widely available.
-  return getTriple().isRISCV() ? 4 : 5;
-}
-
 Tool *ToolChain::getClang() const {
   if (!Clang)
     Clang.reset(new tools::Clang(*this, useIntegratedBackend()));

clang/test/Driver/clang-g-opts.c

@@ -42,8 +42,3 @@
 
 // CHECK-WITH-G-STANDALONE: "-debug-info-kind=standalone"
 // CHECK-WITH-G-STANDALONE: "-dwarf-version=2"
-
-/// TODO: Special case before R_RISCV_SET_ULEB128 linker support becomes more widely available.
-// RUN: %clang -### -S %s -g --target=riscv64-linux-gnu 2>&1 | FileCheck --check-prefix=VERSION4 %s
-// RUN: %clang -### -S %s -g --target=riscv64-unknown-elf 2>&1 | FileCheck --check-prefix=VERSION4 %s
-// VERSION4: "-dwarf-version=4"