add a new TGError class and use it to propagate location info with

errors when thrown.  This gets us nice errors like this from tblgen:

CMOVL32rr: 	(set GR32:i32:$dst, (X86cmov GR32:$src1, GR32:$src2))
/Users/sabre/llvm/Debug/bin/tblgen: error:
Included from X86.td:116:
Parsing X86InstrInfo.td:922: In CMOVL32rr: X86cmov node requires exactly 4 operands!
def CMOVL32rr : I<0x4C, MRMSrcReg,       // if <s, GR32 = GR32
^

instead of just:

CMOVL32rr: 	(set GR32:i32:$dst, (X86cmov GR32:$src1, GR32:$src2))
/Users/sabre/llvm/Debug/bin/tblgen: In CMOVL32rr: X86cmov node requires exactly 4 operands!

This is all I plan to do with this, but it should be easy enough to improve if anyone 
cares (e.g. keeping more loc info in "dag" expr records in tblgen.

llvm-svn: 66898