PseudoSourceValue: Update comments and fix lowercase variable names. NFC.

This commit updates the documentation comments in PseudoSourceValue.cpp and
PseudoSourceValue.h based on the LLVM's documentation style. It also fixes
several instances of variable names that started with a lowercase letter.

This change is done in preparation for the changes to the pseudo source value
object management and to the PseudoSourceValue's class hierarchy.

llvm-svn: 244686