MCParser: Update method names per coding guidelines.

s/AddDirectiveHandler/addDirectiveHandler/
s/ParseMSInlineAsm/parseMSInlineAsm/
s/ParseIdentifier/parseIdentifier/
s/ParseStringToEndOfStatement/parseStringToEndOfStatement/
s/ParseEscapedString/parseEscapedString/
s/EatToEndOfStatement/eatToEndOfStatement/
s/ParseExpression/parseExpression/
s/ParseParenExpression/parseParenExpression/
s/ParseAbsoluteExpression/parseAbsoluteExpression/
s/CheckForValidSection/checkForValidSection/

http://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly

No functional change intended.