Fix comment scanning bug in hash_source_code_string

Fix comment scanning bug in hash_source_code_string

hash_source_code_string tries to ignore __DATE__/__TIME strings in
comments, but fails to parse code that contains character literal of a
double quote. This could result in false cache hits when the source code
happens to contain '"' followed by " /*" or " //" (with variations).

The fix is to do like it's already done in ccache 3.2: Don't try to be
overly clever about __DATE__/__TIME__, just check for those strings
anywhere in the source code string.