Implement a new type of FileID: FileIDInfo::MacroExpansion. For tokens that

came from a macro expansion, this allows us to keep track of both where the
character data came from and where the logical position of the token is (at
the expansion site).  This implements Preprocessor/indent_macro.c, and
reduces the number of cpp iostream -E diffs vs GCC from 2589 to 2297.

llvm-svn: 38557