[SymbolFile] Make ParseCompileUnitXXX accept a CompileUnit&.
Previously all of these functions accepted a SymbolContext&.
While a CompileUnit is one member of a SymbolContext, there
are also many others, and by passing such a monolithic parameter
in this way it makes the requirements and assumptions of the
API unclear for both callers as well as implementors.
All these methods need is a CompileUnit. By limiting the
parameter type in this way, we simplify the code as well as
make it self-documenting for both implementers and users.
Differential Revision: https://reviews.llvm.org/D56564