MIR Parser: Simplify the token's string value handling.

This commit removes the 'StringOffset' and 'HasStringValue' fields from the
MIToken struct and simplifies the 'stringValue' method which now returns
the new 'StringValue' field.

This commit also adopts a different way of initializing the lexed tokens -
instead of constructing a new MIToken instance, the lexer resets the old token
using the new 'reset' method and sets its attributes using the new
'setStringValue', 'setOwnedStringValue', and 'setIntegerValue' methods.

Reviewers: Sean Silva

Differential Revision: http://reviews.llvm.org/D11792

llvm-svn: 244295