[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

llvm-svn: 350943