[AST] Store the string data in StringLiteral in a trailing array of chars

[AST] Store the string data in StringLiteral in a trailing array of chars

Use the newly available space in the bit-fields of Stmt and store the
string data in a trailing array of chars after the trailing array
of SourceLocation. This cuts the size of StringLiteral by 2 pointers.

Also refactor slightly StringLiteral::Create and StringLiteral::CreateEmpty
so that StringLiteral::Create is just responsible for the allocation, and the
constructor is responsible for doing all the initialization. This match what
is done for the other classes in general.

This patch should have no other functional changes apart from this.

A concern was raised during review about the interaction between
this patch and serialization abbreviations. I believe however that
there is currently no abbreviation defined for StringLiteral.
The only statements/expressions which have abbreviations are currently
DeclRefExpr, IntegerLiteral, CharacterLiteral and ImplicitCastExpr.

Differential Revision: https://reviews.llvm.org/D54166

Reviewed By: dblaikie, rjmccall

llvm-svn: 346969