[analyzer] Add CTUImportCppThreshold for C++ files

Summary:
The default CTUImportThreshold (8) seems to be too conservative with C projects.
We increase this value to 24 and we introduce another threshold for C++ source
files (defaulted to 8) because their AST is way more compilcated than C source
files.

Differential Revision: https://reviews.llvm.org/D83475