[libc++] Inline basic_streambuf functions

[philnik777]

Most of the basic_streambuf functions are really simple, which makes most of the implementation when they are out of line boilerplate.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 7c729418d721147bf1f2b257afd30f84721888ad c752280f92a7c2a7b44e788f3f174eba753eff3a --extensions  -- libcxx/include/streambuf

View the diff from clang-format here.

diff --git a/libcxx/include/streambuf b/libcxx/include/streambuf
index 3c05fa26a9..3c4e9086e0 100644
--- a/libcxx/include/streambuf
+++ b/libcxx/include/streambuf
@@ -297,8 +297,7 @@ protected:
 
   // 27.6.2.4.2 Buffer management and positioning:
   virtual basic_streambuf* setbuf(char_type*, streamsize) { return this; }
-  virtual pos_type
-  seekoff(off_type, ios_base::seekdir, ios_base::openmode = ios_base::in | ios_base::out) {
+  virtual pos_type seekoff(off_type, ios_base::seekdir, ios_base::openmode = ios_base::in | ios_base::out) {
     return pos_type(off_type(-1));
   }
   virtual pos_type seekpos(pos_type, ios_base::openmode = ios_base::in | ios_base::out) {