[ASTReader] Sort RawComments before merging

`RawComments` are sorted by comparing underlying `SourceLocation`'s. This is
done by comparing `FileID` and `Offset`; when the `FileID` is the same it means
the locations are within the same TU and the `Offset` is used.

FileID, from the source code: "A mostly-opaque identifier, where 0 is
"invalid", >0 is this module, and <-1 is something loaded from another
module.". That said, when de-serializing SourceLocations, FileID's from
RawComments loaded from other modules get negative IDs where previously they
were positive. This makes imported RawComments unsorted, leading to a wrong
merge with other comments from the current TU. Fix that by sorting RawComments
properly after de-serialization and before merge.

This fixes an assertion in `ASTContext::getRawCommentForDeclNoCache`,
which fires only in a debug build of clang.

Differential Revision: https://reviews.llvm.org/D27546

rdar://problem/29287314

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290134 91177308-0d34-0410-b5e6-96231b3b80d8