Fix assertion on asm register that are "%"

Name might be empty again after we removed the '%' prefix
and Name[0] would assert.

Found on code like
register int foo asm("%" MACRO);
where MACRO was supposed to be defined in a header file that was not found.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215834 91177308-0d34-0410-b5e6-96231b3b80d8