Implement a cpp blacklist for LLVM dependency

This adds a set of private flags that are removed from the output of
`llvm-config --cppflags` before storing them. This is unfortunately
necessary since some versions of LLVM include absolute garbage like
'-DNDEBUG' in their CPP flags, and we don't want to pass those.

mesonbuild/dependencies.py

@@ -1631,6 +1631,7 @@ class LLVMDependency(Dependency):
     llvmconfig = None
     _llvmconfig_found = False
     __best_found = None
+    __cpp_blacklist = {'-DNDEBUG'}
 
     def __init__(self, environment, kwargs):
         super().__init__('llvm-config', kwargs)
@@ -1676,7 +1677,7 @@ class LLVMDependency(Dependency):
             p, out = Popen_safe([self.llvmconfig, '--cppflags'])[:2]
             if p.returncode != 0:
                 raise DependencyException('Could not generate includedir for LLVM.')
-            self.cargs = shlex.split(out)
+            self.cargs = list(mesonlib.OrderedSet(shlex.split(out)).difference(self.__cpp_blacklist))
 
             p, out = Popen_safe([self.llvmconfig, '--components'])[:2]
             if p.returncode != 0: