[llvm][cmake] Follow up to D117973

[Ericson2314]

Summary

1. Slightly document the "mark advanced" variable used to control the installed CMake package dir.
   
   I would document it more, but I am considering in the future adding pkg-config support in this manner, after which _PACKGE_DIR is probably better called _CMAKE_PACKGE_DIR or similar.

2. Convey the custom path to the legacy llvm-config binary.

llvm/docs/CMake.rst

 example) will look for the ``LLVMConfig.cmake`` file in various locations (see
 cmake manual for details).  It creates a ``LLVM_DIR`` cache entry to save the
 directory where ``LLVMConfig.cmake`` is found or allows the user to specify the
 directory (e.g. by passing ``-DLLVM_DIR=/usr/lib/cmake/llvm`` to
 the ``cmake`` command or by setting it directly in ``ccmake`` or ``cmake-gui``).
 
 This file is available in two different locations.
 
-* ``<INSTALL_PREFIX>/lib/cmake/llvm/LLVMConfig.cmake`` where
-  ``<INSTALL_PREFIX>`` is the install prefix of an installed version of LLVM.
-  On Linux typically this is ``/usr/lib/cmake/llvm/LLVMConfig.cmake``.
+* ``<LLVM_INSTALL_PACKAGE_DIR>/LLVMConfig.cmake`` where
+  ``<LLVM_INSTALL_PACKAGE_DIR>`` is the location where LLVM CMake modules are
+  installed as part of an installed version of LLVM. This is typically
+  ``cmake/llvm/`` within the lib directory. On Linux, this is typically
+  ``/usr/lib/cmake/llvm/LLVMConfig.cmake``.
 
 * ``<LLVM_BUILD_ROOT>/lib/cmake/llvm/LLVMConfig.cmake`` where
   ``<LLVM_BUILD_ROOT>`` is the root of the LLVM build tree. **Note: this is only
   available when building LLVM with CMake.**
 
 If LLVM is installed in your operating system's normal installation prefix (e.g.
 on Linux this is usually ``/usr/``) ``find_package(LLVM ...)`` will
 automatically find LLVM if it is installed correctly. If LLVM is not installed

llvm/tools/llvm-config/BuildVariables.inc.in

 #define LLVM_OBJ_ROOT "@LLVM_OBJ_ROOT@"
 #define LLVM_CPPFLAGS "@LLVM_CPPFLAGS@"
 #define LLVM_CFLAGS "@LLVM_CFLAGS@"
 #define LLVM_LDFLAGS "@LLVM_LDFLAGS@"
 #define LLVM_CXXFLAGS "@LLVM_CXXFLAGS@"
 #define LLVM_BUILDMODE "@LLVM_BUILDMODE@"
 #define LLVM_LIBDIR_SUFFIX "@LLVM_LIBDIR_SUFFIX@"
 #define LLVM_INSTALL_INCLUDEDIR "@CMAKE_INSTALL_INCLUDEDIR@"
+#define LLVM_INSTALL_PACKAGE_DIR "@LLVM_INSTALL_PACKAGE_DIR@"
 #define LLVM_TARGETS_BUILT "@LLVM_TARGETS_BUILT@"
 #define LLVM_SYSTEM_LIBS "@LLVM_SYSTEM_LIBS@"
 #define LLVM_BUILD_SYSTEM "@LLVM_BUILD_SYSTEM@"
 #define LLVM_HAS_RTTI @LLVM_HAS_RTTI@
 #define LLVM_ENABLE_DYLIB @LLVM_BUILD_LLVM_DYLIB@
 #define LLVM_LINK_DYLIB @LLVM_LINK_LLVM_DYLIB@
 #define LLVM_ENABLE_SHARED @BUILD_SHARED_LIBS@
 #define LLVM_DYLIB_COMPONENTS "@LLVM_DYLIB_COMPONENTS_expanded@"
 #define LLVM_DYLIB_VERSION "@LLVM_DYLIB_VERSION@"
 #define LLVM_TOOLS_INSTALL_DIR "@LLVM_TOOLS_INSTALL_DIR@"

llvm/tools/llvm-config/llvm-config.cpp

ActivePrefix = CurrentExecPrefix;
       ActiveIncludeDir = std::string(Path.str());
     }
     {
       SmallString<256> Path(LLVM_TOOLS_INSTALL_DIR);
       sys::fs::make_absolute(ActivePrefix, Path);
       ActiveBinDir = std::string(Path.str());
     }
     ActiveLibDir = ActivePrefix + "/lib" + LLVM_LIBDIR_SUFFIX;
-    ActiveCMakeDir = ActiveLibDir + "/cmake/llvm";
+    {
+      SmallString<256> Path(LLVM_INSTALL_PACKAGE_DIR);
+      sys::fs::make_absolute(ActivePrefix, Path);
+      ActiveCMakeDir = std::string(Path.str());
+    }
     ActiveIncludeOption = "-I" + ActiveIncludeDir;
   }
 
   /// We only use `shared library` mode in cases where the static library form
   /// of the components provided are not available; note however that this is
   /// skipped if we're run from within the build dir. However, once installed,
   /// we still need to provide correct output when the static archives are
   /// removed or, as in the case of CMake's `BUILD_SHARED_LIBS`, never present