Fix a parser bug on labeled inline asm stmts, allowing us

Fix a parser bug on labeled inline asm stmts, allowing us
to parse stuff like:

asm volatile("1:  rex64/fxrstor (%[fx])\n\t"
       "2:\n"
       ".section .fixup,\"ax\"\n"
       "3:  movl $-1,%[err]\n"
       "    jmp  2b\n"
       ".previous\n"
       ".section __ex_table,\"a\"\n"
       "   .align 8\n"
       "   .quad  1b,3b\n"
       ".previous"
       : [err] "=r" (err)
       : [fx] "cdaSDb" (fx), "m" (*fx), "0" (0));

This reduces # diagnostics on PR1750 from 49 to 37.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@43434 91177308-0d34-0410-b5e6-96231b3b80d8