[SymbolFile] Remove SymbolContext parameter from FindTypes.

This parameter was only ever used with the Module set, and
since a SymbolFile is tied to a module, the parameter turns
out to be entirely unnecessary.  Furthermore, it doesn't make
a lot of sense to ask a caller to ask SymbolFile which is tied
to Module X to find types for Module Y, but that possibility
was open with the previous interface.  By removing this
parameter from the API, it makes it harder to use incorrectly
as well as easier for an implementor to understand what it
needs to do.

llvm-svn: 351133