Improves support for Unicode in character literals

Updates ProcessUCNExcape() for C++. C++11 allows UCNs in character
and string literals that represent control characters and basic
source characters. Also C++03 allows UCNs that refer to surrogate
codepoints.

UTF-8 sequences in character literals are now handled as single
c-chars.

Added error for multiple characters in Unicode character literals.

Added errors for when a the execution charset encoding of a c-char
cannot be represented as a single code unit in the associated
character type. Note that for the purposes of this error the asso-
ciated character type for a narrow character literal is char, not
int, even though in C narrow character literals have type int.

llvm-svn: 148389