ADT: Add a string APSInt constructor.

This commit moves the APSInt initialization code that's used by
the LLLexer class into a new APSInt constructor that constructs
APSInts from strings.

This change is useful for MIR Serialization, as it would allow
the MILexer class to use the same APSInt initialization as 
LLexer when parsing immediate machine operands.

llvm-svn: 240436