The `CompilerType` is just a wrapper around two pointers, and there is
no usage of the `CompilerType` where those are expected to change
underneath the caller.
To make the interface more straightforward to reason about, this patch
changes all instances of `CompilerType&` to `const CompilerType&` around
the `DWARFASTParserClang` APIs.
We could probably pass these by-value, but all other APIs don't, and
this patch just makes the parameter passing convention consistent with
the rest of the file.
28050e1b0b