introduce a new recoverable error handling API to LLVMContext

and use it in one place in inline asm handling stuff.  Before
we'd generate this for an invalid modifier letter:

$ clang asm.c -c -o t.o
fatal error: error in backend: Invalid operand found in inline asm: 'abc incl    ${0:Z}'
INLINEASM <es:abc incl    ${0:Z}>, 10, %EAX<def>, 2147483657, %EAX, 14, %EFLAGS<earlyclobber,def,dead>, <!-1>


Now we generate this:

$ clang asm.c -c -o t.o
error: invalid operand in inline asm: 'incl    ${0:Z}'
asm.c:3:12: note: generated from here
  __asm__ ("incl    %Z0" : "+r" (X));
           ^
1 error generated.

This is much better but still admittedly not great ("why" is the operand 
invalid??), codegen should try harder with its diagnostics :)

llvm-svn: 100723