Add using directives to BalsaHeader to fix gcc compile error.

Three ConstHeaderApi virtual methods each have two overloads that only differ in
a single argument type: absl::string_view versus QuicheLowerCaseString.  In
Envoy, QuicheLowerCaseString is an alias to Envoy::Http::LowerCaseString [1],
which has an implicit conversion operator to absl::string_view [2].


ConstHeaderApi provides a trivial definition to the QuicheLowerCaseString
overload that calls the absl::string_view overload, which is overridden in
BalsaHeaders.  This makes perfect sense but makes gcc in Envoy unhappy:

quiche/common/balsa/header_api.h:89:23: error:
‘virtual std::string quiche::ConstHeaderApi::GetAllOfHeaderAsString(const QuicheLowerCaseString&) const’ was hidden [-Werror=overloaded-virtual]
quiche/common/balsa/balsa_headers.h:560:15: error:
by ‘virtual std::string quiche::BalsaHeaders::GetAllOfHeaderAsString(absl::string_view) const’ [-Werror=overloaded-virtual]

This is because when called with a QuicheLowerCaseString argument on a
BalsaHeaders object, the latter overload is preferred over the former (due to
the implicit conversion), which is not the intended behavior.

GetAllOfHeader() does not pose an issue due to the
  using HeaderApi::GetAllOfHeader;
directive at balsa_headers.h:549.

This CL adds similar using directives to the other two methods to solve this

And here is a minimal example to verify this behavior.  Running with
gcc -Woverloaded-virtual -l:libstdc++.a && ./a.out
prints "right" and no warning, but if the using directive is removed, the
warning appears and "wrong" is printed:

#include <iostream>

class T {
  operator int() const { return 0; }

class base {
  virtual void print(int a) = 0;
  virtual void print(T a) { std::cout << "right" << std::endl; }

class derived : public base {
  using base::print;
  void print(int a) override { std::cout << "wrong" << std::endl; }

int main() {
  T t;
  derived c;
  return 0;

PiperOrigin-RevId: 448474666
diff --git a/quiche/balsa/balsa_headers.h b/quiche/balsa/balsa_headers.h
index ad51cb8..9302dad 100644
--- a/quiche/balsa/balsa_headers.h
+++ b/quiche/balsa/balsa_headers.h
@@ -517,6 +517,9 @@
   // Returns the value corresponding to the given header key. Returns an empty
   // string if the header key does not exist. For headers that may consist of
   // multiple lines, use GetAllOfHeader() instead.
+  // Make the QuicheLowerCaseString overload visible,
+  // and only override the absl::string_view one.
+  using HeaderApi::GetHeader;
   absl::string_view GetHeader(absl::string_view key) const override;
   // Iterates over all currently valid header lines, appending their
@@ -545,7 +548,8 @@
   // See gfe::header_properties::IsMultivaluedHeader() for which headers
   // GFE treats as being multivalued.
-  // Make all methods in this overload set visible, and override just one.
+  // Make the QuicheLowerCaseString overload visible,
+  // and only override the absl::string_view one.
   using HeaderApi::GetAllOfHeader;
   void GetAllOfHeader(absl::string_view key,
                       std::vector<absl::string_view>* out) const override;
@@ -557,6 +561,9 @@
                                     std::vector<absl::string_view>* out) const;
   // Joins all values for `key` into a comma-separated string.
+  // Make the QuicheLowerCaseString overload visible,
+  // and only override the absl::string_view one.
+  using HeaderApi::GetAllOfHeaderAsString;
   std::string GetAllOfHeaderAsString(absl::string_view key) const override;
   // Determine if a given header is present.  Case-insensitive.