[build] Fix stand-alone builds of clang.

The header file `llvm/include/llvm/Targetparser/RISCVTargetParser.h`
relies on the auto-generated *.inc file associated to the tablegen
target `RISCVTargetParserTableGen`.

Both clangBasic and clangDriver include `RISCVTargetParser.h`,
therefore we need to make sure that the *.inc file is avaiable to
avoid compilation errors like the following:

    FAILED: tools/clang/lib/Basic/CMakeFiles/obj.clangBasic.dir/Targets/RISCV.cpp.o
    /usr/bin/c++  [bunch of non interesting stuff]  -c <path-to>/llvm-project/clang/lib/Basic/Targets/RISCV.cpp
    In file included from <path-to>/llvm-project/clang/lib/Basic/Targets/RISCV.cpp:19:
    <path-to>/llvm-project/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"
         |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The stand-alone build of `clang` has been tested with the following
script (see [*] for further information):

```
build_llvm=`pwd`/build-llvm
build_clang=`pwd`/build-clang
installprefix=`pwd`/install
llvm=`pwd`/llvm-project
mkdir -p $build_llvm
mkdir -p $installprefix

cmake -G Ninja -S $llvm/llvm -B $build_llvm \
      -DLLVM_INSTALL_UTILS=ON \
      -DCMAKE_INSTALL_PREFIX=$installprefix \
      -DCMAKE_BUILD_TYPE=Release

ninja -C $build_llvm install

cmake -G Ninja -S $llvm/clang -B $build_clang \
      -DLLVM_EXTERNAL_LIT=$build_llvm/utils/lit \
      -DLLVM_ROOT=$installprefix
```

[*] https://llvm.org/docs/GettingStarted.html#stand-alone-builds

Differential Revision: https://reviews.llvm.org/D141581