AsmParser: Extend the API to make the global value and metadata node slot mappings...

AsmParser: Extend the API to make the global value and metadata node slot mappings publicly accessible.

This commit creates a new structure called 'SlotMapping' in the AsmParser library.
This structure can be passed into the public parsing APIs from the AsmParser library
in order to extract the data structures that map from slot numbers to unnamed global
values and metadata nodes.

This change is useful for MIR Serialization, as the MIR Parser has to lookup the
unnamed global values and metadata nodes by their slot numbers.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10551

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240427 91177308-0d34-0410-b5e6-96231b3b80d8