[flang] Use c_str() rather than data() where NUL termination is assumed

Original-commit: flang-compiler/f18@5baed3c856
Reviewed-on: https://github.com/flang-compiler/f18/pull/804

flang/documentation/C++style.md

@@ -186,6 +186,8 @@ in `if`/`while` predicates.
 When no presence test is obviously protecting a `*x` reference to the
 contents, and it is assumed that the contents are present, validate that
 assumption by using `x.value()` instead.
+1. We use `c_str()` rather than `data()` when converting a `std::string`
+to a `const char *` when the result is expected to be NUL-terminated.
 
 #### Classes
 1. Define POD structures with `struct`.

flang/lib/evaluate/check-expression.cc

@@ -18,8 +18,6 @@
 #include "../semantics/symbol.h"
 #include "../semantics/tools.h"
 
-using namespace std::literals::string_literals;
-
 namespace Fortran::evaluate {
 
 // Constant expression predicate IsConstantExpr().

flang/lib/evaluate/type.cc

@@ -27,8 +27,6 @@
 #include <sstream>
 #include <string>
 
-using namespace std::literals::string_literals;
-
 // IsDescriptor() predicate
 // TODO there's probably a better place for this predicate than here
 namespace Fortran::semantics {

flang/lib/parser/dump-parse-tree.h

@@ -25,8 +25,6 @@
 
 namespace Fortran::parser {
 
-using namespace std::string_literals;
-
 // When SHOW_ALL_SOURCE_MEMBERS is defined, HasSource<T>::value is true if T has
 // a member named source
 template<typename T, typename = int> struct HasSource : std::false_type {};

flang/lib/parser/prescan.cc

@@ -1081,7 +1081,7 @@ const char *Prescanner::IsCompilerDirectiveSentinel(
     return nullptr;
   }
   const auto iter{compilerDirectiveSentinels_.find(std::string(sentinel, n))};
-  return iter == compilerDirectiveSentinels_.end() ? nullptr : iter->data();
+  return iter == compilerDirectiveSentinels_.end() ? nullptr : iter->c_str();
 }
 
 Prescanner::LineClassification Prescanner::ClassifyLine(