[FileCheck] Fix PR49531: invalid use of string var

FileCheck string substitution block parsing code only report an invalid
variable name in a string variable use if it starts with a forbidden
character. It does not report anything if there are unparsed characters
after the variable name, i.e. [[X-Y]] is parsed as [[X]] and no error is
returned. This commit fixes that.

Reviewed By: jdenny, jhenderson

Differential Revision: https://reviews.llvm.org/D98691