AsmParser: Add a function to parse a standalone constant value.

This commit extends the interface provided by the AsmParser library by adding a
function that allows the user to parse a standalone contant value.

This change is useful for MIR serialization, as it will allow the MIR Parser to
parse the constant values in a machine constant pool.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10280

llvm-svn: 242579