[CMake] Deprecate GCC_INSTALL_PREFIX (#77537)

Part of https://reviews.llvm.org/D158218

GCC_INSTALL_PREFIX is a rarely-used legacy option inherited from
pre-CMake build system and has configuration file replacement nowadays.
Many `clang/test/Driver` tests specify `--gcc-toolchain=` to prevent
failures when `GCC_INSTALL_PREFIX` is specified: some contributors add
them to fix tests and some just do cargo culting. This is not healthy
for contributors adding cross compilation support for this rarely used
option.

`DEFAULT_SYSROOT` should in spirit be deprecated as well, but a relative
path doesn't have good replacement, so don't deprecate it for now.

Link:
https://discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link:
https://discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833

---

With `GCC_INSTALL_PREFIX=/usr`, `clang a.c` behaves like
`clang --gcc-toolchain=/usr a.c`.

Here is a simplified version of GCC installation detection code.
```
if (OPT_gcc_install_dir_EQ)
  return OPT_gcc_install_dir_EQ;

if (OPT_gcc_triple)
  candidate_gcc_triples = {OPT_gcc_triple};
else
  candidate_gcc_triples = collectCandidateTriples();
if (OPT_gcc_toolchain)
  prefixes = {OPT_gcc_toolchain};
else
  prefixes = {OPT_sysroot/usr, OPT_sysroot};
for (prefix : prefixes)
  if "$prefix/lib/gcc" exists // also tries $prefix/lib/gcc-cross
    for (triple : candidate_gcc_triples)
      if "$prefix/lib/gcc/$triple" exists
        return "$prefix/lib/gcc/$triple/$version"; // pick the largest version
```

`--gcc-toolchain=` specifies a directory where
`lib/gcc{,-cross}/$triple/$version` can be found. If you actually want
to use a specific version of GCC, specify something like
`--gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/11` in a configuration
file. You can also specify `--gcc-triple=`.

On Debian and its derivatives where the target triple omits the vendor
part, the following ways are roughly equivalent, except that
`--gcc-install-dir=` specifies a version as well:
```
clang --gcc-toolchain=/usr a.c
clang --gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/11 a.c
clang --gcc-triple=x86_64-linux-gnu a.c
```

Author: MaskRay

clang/CMakeLists.txt

@@ -193,6 +193,12 @@ set(C_INCLUDE_DIRS "" CACHE STRING
 set(GCC_INSTALL_PREFIX "" CACHE PATH "Directory where gcc is installed." )
 set(DEFAULT_SYSROOT "" CACHE STRING
   "Default <path> to all compiler invocations for --sysroot=<path>." )
+if(GCC_INSTALL_PREFIX)
+  message(WARNING "GCC_INSTALL_PREFIX is deprecated and will be removed. Use "
+    "configuration files (https://clang.llvm.org/docs/UsersManual.html#configuration-files)"
+    "to specify the default --gcc-install-dir= or --gcc-triple=. --gcc-toolchain= is discouraged. "
+    "See https://github.com/llvm/llvm-project/pull/77537 for detail.")
+endif()
 
 set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
 

clang/docs/ReleaseNotes.rst

@@ -37,6 +37,12 @@ These changes are ones which we think may surprise users when upgrading to
 Clang |release| because of the opportunity they pose for disruption to existing
 code bases.
 
+- The CMake variable ``GCC_INSTALL_PREFIX`` (which sets the default
+  ``--gcc-toolchain=``) is deprecated and will be removed. Specify
+  ``--gcc-install-dir=`` or ``--gcc-triple=`` in a `configuration file
+  <https://clang.llvm.org/docs/UsersManual.html#configuration-files>` as a
+  replacement.
+  (`#77537 <https://github.com/llvm/llvm-project/pull/77537>`_)
 
 C/C++ Language Potentially Breaking Changes
 -------------------------------------------