For Clang synthesized __va_list_tag (CreateX86_64ABIBuiltinVaListDecl),
its DW_AT_decl_file/DW_AT_decl_line are arbitrarily set from CurLoc.
In a stage 2 -DCMAKE_BUILD_TYPE=Debug clang build, I observe that
in driver.cpp, DW_AT_decl_file/DW_AT_decl_line may be set to an #include line
(the transitively included file uses va_arg (__builtin_va_arg)).
This seems arbitrary. Drop that.
Depends on D94391