Fix a weird inconsistency with hex floats. Previously the lexer

Fix a weird inconsistency with hex floats.  Previously the lexer
would not eat the "-1" in "0x0p-1", but LiteralSupport would accept
it when extensions are on.  This caused strangeness and failures
when hexfloats were properly treated as an extension (not error)
in LiteralSupport.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59865 91177308-0d34-0410-b5e6-96231b3b80d8