optimized TokenList::readfile() a bit

[firewave]

Testing with -q -Ilib/ -D__GNUC__ lib/valueflow.cpp:

Clang 14 271,886,478 -> 259,691,836
GCC 12 275,880,976 -> 262,961,857

Testing with -q -Ilib/ -D__GNUC__ lib/tokenize.cpp:

Clang 14 252,880,452 -> 241,542,054
GCC 12 257,203,604 -> 245,222,755

[firewave]

Visual Studio asserts with cannot seek string iterator before begin for the std::equal() usage.

[firewave]

Visual Studio asserts with cannot seek string iterator before begin for the std::equal() usage.

What a coincidence: llvm/llvm-project@1c5ad6d