ClangIR CodeGen code uses both `mlir::MLIRContext` and
`clang::ASTContext` objects extensively. Refering to either of those as
just "context" can be confusing.
Change the names of all variables, parameters, and fields in
`clang/lib/CIR/CodeGen` that refer to `MLIRContext` or an `ASTContext`
to be either `mlirContext` or `astContext`.
This change is only the renaming of variables/parameters/fields. There
are no behavior changes. So there are no new tests or changes to
existing tests. This change mimics a recent change in the ClangIR
incubator repository.
7eb73b95cb