Only use major version in the clang resource directory for standalone builds.

Commit e1b88c8 changed the name of the clang resource directory so that
it was "lib/clang/<MajorVersion>" but missed the place in the LLDB standalone
build where we search for the resource directory.  That was still looking for
<Major>.<Minor>.<Patch>.  The standalone lldb bot has been failing since this
commit.

Author: jimingham

lldb/cmake/modules/LLDBConfig.cmake

@@ -271,7 +271,7 @@ endif()
 # directory that LLDB is using for its embedded Clang instance needs to point
 # to the resource directory of the used Clang installation.
 if (NOT TARGET clang-resource-headers)
-  set(LLDB_CLANG_RESOURCE_DIR_NAME "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}")
+  set(LLDB_CLANG_RESOURCE_DIR_NAME "${LLVM_VERSION_MAJOR}")
   # Iterate over the possible places where the external resource directory
   # could be and pick the first that exists.
   foreach(CANDIDATE "${Clang_DIR}/../.." "${LLVM_DIR}" "${LLVM_LIBRARY_DIRS}"