[CMake] Use LLVM_DIR and Clang_DIR for standalone builds.

When doing a standalone build, without setting LLDB_PATH_TO_LLVM_BUILD
or LLDB_PATH_TO_CLANG_BUILD, you get the following error.

```
CMake Error at cmake/modules/LLDBStandalone.cmake:23 (find_package):
  Could not find a package configuration file provided by "LLVM" with any of
  the following names:

    LLVMConfig.cmake
    llvm-config.cmake

  Add the installation prefix of "LLVM" to CMAKE_PREFIX_PATH or set
  "LLVM_DIR" to a directory containing one of the above files.  If "LLVM"
  provides a separate development package or SDK, be sure it has been
  installed.
```

This suggests setting LLVM_DIR to LLVM's install directory. However,
LLDBStandalone.cmake takes LLDB_PATH_TO_LLVM_BUILD as its hint. As
someone who isn't familiar with the standalone process, this is rather
confusing. This patch removes LLDB_PATH_TO_LLVM_BUILD and
LLDB_PATH_TO_CLANG_BUILD and instead use LLVM_DIR and Clang_DIR
respectively.

Differential revision: https://reviews.llvm.org/D64823

llvm-svn: 366362