[clang] Add gcc-toolset-10 support (RHEL/CentOS 8)

Clang only adds GCC paths for RHEL <= 7 'devtoolset-<N>' Software
Collections (SCL).  This generalizes this support to also include the
'gcc-toolset-10' SCL in RHEL/CentOS 8.

Reviewed By: stephan.dollberg

Differential Revision: https://reviews.llvm.org/D108908

Author: Simon Moll

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -2052,7 +2052,8 @@ void Generic_GCC::GCCInstallationDetector::AddDefaultGCCPrefixes(
 
   // Non-Solaris is much simpler - most systems just go with "/usr".
   if (SysRoot.empty() && TargetTriple.getOS() == llvm::Triple::Linux) {
-    // Yet, still look for RHEL devtoolsets.
+    // Yet, still look for RHEL/CentOS devtoolsets and gcc-toolsets.
+    Prefixes.push_back("/opt/rh/gcc-toolset-10/root/usr");
     Prefixes.push_back("/opt/rh/devtoolset-10/root/usr");
     Prefixes.push_back("/opt/rh/devtoolset-9/root/usr");
     Prefixes.push_back("/opt/rh/devtoolset-8/root/usr");

clang/lib/Driver/ToolChains/Linux.cpp

@@ -206,8 +206,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
     ExtraOpts.push_back("max-page-size=4096");
   }
 
-  if (GCCInstallation.getParentLibPath().find("opt/rh/devtoolset") !=
-      StringRef::npos)
+  if (GCCInstallation.getParentLibPath().find("opt/rh/") != StringRef::npos)
     // With devtoolset on RHEL, we want to add a bin directory that is relative
     // to the detected gcc install, because if we are using devtoolset gcc then
     // we want to use other tools from devtoolset (e.g. ld) instead of the