Introduce a global bit-offset continuous range map into the ASTReader,

so that we have one, simple way to map from global bit offsets to
local bit offsets. Eliminates a number of loops over the chain, and
generalizes for more interesting bit remappings.

Also, as an amusing oddity, we were computing global bit offsets
*backwards* for preprocessed entities (e.g., the directly included PCH
file in the chain would start at offset zero, rather than the original
PCH that occurs first in translation unit). Even more amusingly, it
made precompiled preambles work, because we were forgetting to adjust
the local bit offset to a global bit offset when storing preprocessed
entity offsets in the ASTUnit. Two wrongs made a right, and now
they're both right.

llvm-svn: 135750