[clang] fix out of bounds access in an empty string when lexing a _Pragma with missing string token

The lexer can attempt to lex a _Pragma and crash with an out of bounds string access when it's
lexing a _Pragma whose string token is an invalid buffer, e.g. when a module header file from which the macro
expansion for that token was deleted from the file system.

Differential Revision: https://reviews.llvm.org/D116052