gnu: clang@7, clang@8: Do not refer to /lib on foreign distros.

Partly fixes <https://issues.guix.gnu.org/51088>.

* gnu/packages/patches/clang-7.0-libc-search-path.patch: Update.
* gnu/packages/patches/clang-8.0-libc-search-path.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/llvm.scm (clang-8): Use it.

gnu/packages/patches/clang-8.0-libc-search-path.patch

@@ -0,0 +1,94 @@
+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/lib/Driver/Distro.cpp b/lib/Driver/Distro.cpp
+index 396d0bee..90e06613 100644
+--- a/lib/Driver/Distro.cpp
++++ b/lib/Driver/Distro.cpp
+@@ -19,6 +19,10 @@ using namespace clang::driver;
+ using namespace clang;
+ 
+ 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;
++
+   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> File =
+       VFS.getBufferForFile("/etc/lsb-release");
+   if (File) {
+diff --git a/lib/Driver/ToolChains/Cuda.cpp b/lib/Driver/ToolChains/Cuda.cpp
+index 57b8d434..57339f1a 100644
+--- a/lib/Driver/ToolChains/Cuda.cpp
++++ b/lib/Driver/ToolChains/Cuda.cpp
+@@ -68,6 +68,9 @@ 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/lib/Driver/ToolChains/Linux.cpp b/lib/Driver/ToolChains/Linux.cpp
+index 65ab9b2d..5ed28e0a 100644
+--- a/lib/Driver/ToolChains/Linux.cpp
++++ b/lib/Driver/ToolChains/Linux.cpp
+@@ -233,7 +233,9 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+   PPaths.push_back(Twine(GCCInstallation.getParentLibPath() + "/../" +
+                          GCCInstallation.getTriple().str() + "/bin")
+                        .str());
+-
++  // 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());
+ 
+   if (Distro.IsAlpineLinux() || Triple.isAndroid()) {
+@@ -304,6 +306,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+ 
+   if (IsAndroid || Distro.IsOpenSUSE())
+     ExtraOpts.push_back("--enable-new-dtags");
++#endif
+ 
+   // The selection of paths to try here is designed to match the patterns which
+   // the GCC driver itself uses, as this is part of the GCC-compatible driver.
+@@ -366,6 +369,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+     }
+   }
+ 
++#if 0
+   // Similar to the logic for GCC above, if we currently running Clang inside
+   // of the requested system root, add its parent library paths to
+   // those searched.
+@@ -411,6 +415,8 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+   // Try walking via the GCC triple path in case of biarch or multiarch GCC
+   // installations with strange symlinks.
+   if (GCCInstallation.isValid()) {
++    // The following code would end up adding things like
++    // "/usr/lib/x86_64-unknown-linux-gnu/../../lib64" to the search path.
+     addPathIfExists(D,
+                     SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
+                         "/../../" + OSLibDir,
+@@ -446,9 +452,14 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+   // directory ('Dir' below) or the ResourceDir.
+   if (StringRef(D.Dir).startswith(SysRoot))
+     addPathIfExists(D, D.Dir + "/../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);
+ 
+-  addPathIfExists(D, SysRoot + "/lib", Paths);
+-  addPathIfExists(D, SysRoot + "/usr/lib", Paths);
++  // Add GCC's lib/ directory so libstdc++.so can be found.
++  addPathIfExists(D, GCCInstallation.getParentLibPath(), Paths);
+ }
+ 
+ ToolChain::CXXStdlibType Linux::GetDefaultCXXStdlibType() const {