Move CheckForValidSection to the MCAsmParser interface.

Now that it behaves itself in terms of streamer independence (r172450), this
method can be moved to MCAsmParser to be available to all extensions,
overriding, etc.

-- -This line, and those below, will be ignored--

M    lib/MC/MCParser/AsmParser.cpp
M    include/llvm/MC/MCParser/MCAsmParser.h