Revert "[build] Fix stand-alone builds of clang."

It breaks some builds [1] with the following error:

```
ccache /usr/bin/c++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.build/tools/clang/lib/Basic -I/home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.src/clang/lib/Basic -I/home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.src/clang/include -I/home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.build/tools/clang/include -I/home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.build/include -I/home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.src/llvm/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -fdiagnostics-color -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -fno-strict-aliasing -O3 -DNDEBUG  -fno-exceptions -fno-rtti -UNDEBUG -std=c++17 -MD -MT tools/clang/lib/Basic/CMakeFiles/obj.clangBasic.dir/Targets/RISCV.cpp.o -MF tools/clang/lib/Basic/CMakeFiles/obj.clangBasic.dir/Targets/RISCV.cpp.o.d -o tools/clang/lib/Basic/CMakeFiles/obj.clangBasic.dir/Targets/RISCV.cpp.o -c /home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.src/clang/lib/Basic/Targets/RISCV.cpp
In file included from /home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.src/clang/lib/Basic/Targets/RISCV.cpp:19:
/home/omp-vega20-0/bbot/openmp-offload-amdgpu-runtime/llvm.src/llvm/include/llvm/TargetParser/RISCVTargetParser.h:29:10: fatal error: llvm/TargetParser/RISCVTargetParserDef.inc: No such file or directory
   29 | #include "llvm/TargetParser/RISCVTargetParserDef.inc"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
2.225 [3029/31/825] Building CXX object tools/clang/lib/Basic/CMakeFiles/obj.clangBasic.dir/Version.cpp.o
2.524 [3029/30/826] Building RISCVTargetParserDef.inc...
```

[1] https://lab.llvm.org/buildbot/#/builders/193/builds/25362

This reverts commit 52bcdac3.