PseudoSourceValue: Transform the mips subclass to target independent subclasses

This commit transforms the mips-specific 'MipsCallEntry' subclass of the
'PseudoSourceValue' class into two, target-independent subclasses named
'GlobalValuePseudoSourceValue' and 'ExternalSymbolPseudoSourceValue'.

This change makes it easier to serialize the pseudo source values by removing
target-specific pseudo source values.

Reviewers: Akira Hatanaka
llvm-svn: 244698