Update parser's disambiguation to cope with braced function-style casts in

C++11, and with braced-init-list initializers in conditions. This exposed an
ambiguity with enum underlying types versus bitfields, which we resolve by
treating 'enum E : T {' as always defining an enumeration (even if it would
only successfully parse as a bitfield). This appears to be g++ compatible.

llvm-svn: 151227