gnu: Add llvm-18.

* gnu/packages/llvm.scm (clang-runtime-from-llvm)[native-inputs]: When
building clang-runtime-18 or newer use gcc-13.
(%llvm-monorepo-hashes): Add entry for llvm-18.
(%llvm-patches): Add entry for llvm-18.
(llvm-18, clang-runtime-18, clang-18, libomp-18, clang-toolchain-18):
New variables.
* gnu/packages/patches/clang-18.0-libc-search-path.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.

Change-Id: I4027065398300104c6a310187007448f5b2780c1

gnu/packages/patches/clang-18.0-libc-search-path.patch

@@ -0,0 +1,90 @@
+Clang attempts to guess file names based on the OS and distro (yes!),
+but unfortunately, that doesn't work for us.
+
+This patch makes it easy to insert libc's $libdir so that Clang passes the
+correct absolute file name of crt1.o etc. to 'ld'.  It also disables all
+the distro-specific stuff and removes the hard-coded FHS directory names
+to make sure Clang also works on foreign distros.
+
+diff --git a/clang/lib/Driver/Distro.cpp b/clang/lib/Driver/Distro.cpp
+index a7e7f169dc14..0564003352a4 100644
+--- a/clang/lib/Driver/Distro.cpp
++++ b/clang/lib/Driver/Distro.cpp
+@@ -100,6 +100,10 @@ static Distro::DistroType DetectLsbRelease(llvm::vfs::FileSystem &VFS) {
+ }
+ 
+ static Distro::DistroType DetectDistro(llvm::vfs::FileSystem &VFS) {
++  // The compiler should always behave the same, even when used via Guix on a
++  // foreign distro.
++  return Distro::UnknownDistro;
++
+   Distro::DistroType Version = Distro::UnknownDistro;
+ 
+   // Newer freedesktop.org's compilant systemd-based systems
+diff --git a/clang/lib/Driver/ToolChains/Cuda.cpp b/clang/lib/Driver/ToolChains/Cuda.cpp
+index 1462576ca870..1078dba8f481 100644
+--- a/clang/lib/Driver/ToolChains/Cuda.cpp
++++ b/clang/lib/Driver/ToolChains/Cuda.cpp
+@@ -132,6 +132,9 @@ CudaInstallationDetector::CudaInstallationDetector(
+     const Driver &D, const llvm::Triple &HostTriple,
+     const llvm::opt::ArgList &Args)
+     : D(D) {
++  // Don't look for CUDA in /usr.
++  return;
++
+   struct Candidate {
+     std::string Path;
+     bool StrictChecking;
+diff --git a/clang/lib/Driver/ToolChains/Linux.cpp b/clang/lib/Driver/ToolChains/Linux.cpp
+index 4300a2bdff17..ac3b29d0fc13 100644
+--- a/clang/lib/Driver/ToolChains/Linux.cpp
++++ b/clang/lib/Driver/ToolChains/Linux.cpp
+@@ -224,6 +224,10 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+ 
+   Generic_GCC::PushPPaths(PPaths);
+ 
++  // Comment out the distro-specific tweaks so that they don't bite when
++  // using Guix on a foreign distro.
++#if 0
++
+   Distro Distro(D.getVFS(), Triple);
+ 
+   if (Distro.IsAlpineLinux() || Triple.isAndroid()) {
+@@ -284,6 +288,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+ 
+ #ifdef ENABLE_LINKER_BUILD_ID
+   ExtraOpts.push_back("--build-id");
++#endif
+ #endif
+ 
+   // The selection of paths to try here is designed to match the patterns which
+@@ -305,6 +310,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+   }
+   Generic_GCC::AddMultilibPaths(D, SysRoot, OSLibDir, MultiarchTriple, Paths);
+ 
++#if 0
+   addPathIfExists(D, concat(SysRoot, "/lib", MultiarchTriple), Paths);
+   addPathIfExists(D, concat(SysRoot, "/lib/..", OSLibDir), Paths);
+ 
+@@ -333,11 +339,21 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+     addPathIfExists(D, concat(SysRoot, "/", OSLibDir, ABIName), Paths);
+     addPathIfExists(D, concat(SysRoot, "/usr", OSLibDir, ABIName), Paths);
+   }
++#endif
+ 
+   Generic_GCC::AddMultiarchPaths(D, SysRoot, OSLibDir, Paths);
+ 
++#if 0
+   addPathIfExists(D, concat(SysRoot, "/lib"), Paths);
+   addPathIfExists(D, concat(SysRoot, "/usr/lib"), Paths);
++#endif
++
++  // Add libc's lib/ directory to the search path, so that crt1.o, crti.o,
++  // and friends can be found.
++  addPathIfExists(D, "@GLIBC_LIBDIR@", Paths);
++
++  // Add GCC's lib/ directory so libstdc++.so can be found.
++  addPathIfExists(D, GCCInstallation.getParentLibPath(), Paths);
+ }
+ 
+ ToolChain::RuntimeLibType Linux::GetDefaultRuntimeLibType() const {