Fix undefined behavior (signed integer overflow) when Clang parses a hexfloat...

Fix undefined behavior (signed integer overflow) when Clang parses a hexfloat with an enormous exponent. Caught by an existing unit test + -ftrapv.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162505 91177308-0d34-0410-b5e6-96231b3b80d8