Skip leading spaces when decoding hex values

Summary:
The StringExtractor functions using stroull will already
skip leading whitespace (ie GetU64). Make sure that the manual
hex parsing functions also skip leading whitespace.

This is important for members of the gdb protocol which are defined
as using whitespace separators (ie qfThreadInfo, qC, etc). While
lldb-server does not use the whitespace separators, gdb-remotes
should work if they do, as the whitespace is defined by the gdb-remote
protocol.

Reviewers: vharron, jasonmolenda, clayborg

Subscribers: sas, lldb-commits

Differential Revision: http://reviews.llvm.org/D20509

llvm-svn: 270592