Reapply r112091 and r111922, support for metadata linking, with a

fix: add a flag to MapValue and friends which indicates whether
any module-level mappings are being made. In the common case of
inlining, no module-level mappings are needed, so MapValue doesn't
need to examine non-function-local metadata, which can be very
expensive in the case of a large module with really deep metadata
(e.g. a large C++ program compiled with -g).

This flag is a little awkward; perhaps eventually it can be moved
into the ClonedCodeInfo class.

llvm-svn: 112190

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