Modification of GNU-Compiler to support UTF16-String literals

Purpose

Support u'c' and u"UTF16 string literal" analogue to L'c' and L"wide
string literal".

Specification
(see chapter 6.1.4 "String literals" of the C89 standard)

u-string-literal:	
u"s-char-sequenceopt"

s-char-sequence:
	s-char
	s-char-sequence s-char

s-char:
	any member of the source character set except the double-quote ",
        backslash \, or new-line character escape-sequence


Implementation

Our approach was to search all places where L-literals where handled
explicitly and to add analogue coding for UTF16 string literals.

Step 1: Scanning u-literals

We identified the place in the compiler where L-literals where handled
by the scanner and added analogue handling for the u-literals.  As
type for the scanned u-literals we used an alias of type unsigned
short or unsigned short array, respectively.

Step 2: Parser

In the parser there was only one location where special handling of
L-literals occurred: the routine that concatenated several strictly
adjacent literals into one literal.

Step 3: Semantic analysis

In the semantic analysis we found three places where special handling
of L-literals occurred: 
- the check, if char/wide char pointers are initialized with a string
  literal of the proper type
- the permission to convert string literals implicitly to non-const
  char/wide char pointer
- the initialization of string arrays without the terminating 0 of the
  string literal (only allowed in C)

Step 4: Code generation

In the code generation and optimization there was no location, where
special handling of L-literals occurred.