Support floating point values in 128-bit SSE vector registers

The System-V x86_64 ABI requires floating point values to be passed
in 128-but SSE vector registers (xmm0, ...). When printing such a
variable this currently yields an <invalid load address>.

This patch makes LLDB's DWARF expression evaluator accept 128-bit
registers as scalars. It also relaxes the check that the size of the
result of the DWARF expression be equal to the size of the variable to a
greater-than. DWARF defers to the ABI how smaller values are being placed
in a larger register.

Implementation note: I found the code in Value::SetContext() that changes
the m_value_type after the fact to be questionable. I added a sanity check
that the Value's memory buffer has indeed been written to (this is
necessary, because we may have a scalar value in a vector register), but
really I feel like this is the wrong place to be setting it.

Reviewed by Greg Clayton.

http://reviews.llvm.org/D17897
rdar://problem/24944340

llvm-svn: 262947