Do not parse hexadecimal floating point literals in C++0x mode because they are

incompatible with user-defined literals, specifically with the following form:

  0x1p+1

The preprocessing-number token extends only as far as the 'p'; the '+' is not
included. Previously we could get away with this extension as p was an invalid
suffix, but now with user-defined literals, 'p' might well be a valid suffix
and we are forced to consider it as such.

This patch also adds a warning in non-0x C++ modes telling the user that
this extension is incompatible with C++0x that is enabled by default
(previously and with other languages, we warn only with a compliance
option such as -pedantic).

llvm-svn: 93135