Revert "meson: Add support for using cmake for finding LLVM"

This reverts commit 5157a42765.

There is a meson bug that causes llvm to always be statically linked,
which is obviously not what we want. I haven't had time to look into it
yet, but for now let's just revert it.

meson.build

@@ -1264,6 +1264,7 @@ if _llvm != 'false'
       with_gallium_opencl or _llvm == 'true'
     ),
     static : not _shared_llvm,
+    method : 'config-tool',
   )
   with_llvm = dep_llvm.found()
 endif
@@ -1277,17 +1278,7 @@ if with_llvm
   # LLVM can be built without rtti, turning off rtti changes the ABI of C++
   # programs, so we need to build all C++ code in mesa without rtti as well to
   # ensure that linking works.
-  #
-  # In meson 0.51.0 we can use cmake to find LLVM in addittion to meson's
-  # builtin llvm-config based finder. A new generic variable getter method
-  # has also been added, so we'll use that if we can, to cover the cmake case.
-  if meson.version().version_compare('>=0.51')
-    # The CMake finder will return 'ON', the llvm-config will return 'YES'
-    _rtti = ['ON', 'YES'].contains(dep_llvm.get_variable(cmake : 'LLVM_ENABLE_RTTI', configtool: 'has-rtti'))
-  else
-    _rtti = dep_llvm.get_configtool_variable('has-rtti') == 'YES'
-  endif
-  if not _rtti
+  if dep_llvm.get_configtool_variable('has-rtti') == 'NO'
     if with_gallium_nouveau
       error('The Nouveau driver requires rtti. You either need to turn off nouveau or use an LLVM built with LLVM_ENABLE_RTTI.')
     elif with_gallium_opencl