[Backtracing][Linux] Move doc comment to header.

Moved the comment for `_swift_backtrace_demangle` into the header file
instead of it being in the implementation.

rdar://110261430

Author: al45tair

include/swift/Runtime/Backtrace.h

@@ -136,6 +136,33 @@ SWIFT_RUNTIME_STDLIB_INTERNAL BacktraceSettings _swift_backtraceSettings;
 
 SWIFT_RUNTIME_STDLIB_SPI SWIFT_CC(swift) bool _swift_isThunkFunction(const char *mangledName);
 
+/// Try to demangle a symbol.
+///
+/// Unlike other entry points that do this, we try both Swift and C++ here.
+///
+/// @param mangledName is the symbol name to be demangled.
+/// @param mangledNameLength is the length of this name.
+/// @param outputBuffer is a pointer to a buffer in which to place the result.
+/// @param outputBufferSize points to a variable that contains the size of the
+/// output buffer.
+/// @param status returns the status codes defined in the C++ ABI.
+///
+/// If outputBuffer is nullptr, the function will allocate memory for the
+/// result using malloc().  In this case, outputBufferSize may be nullptr;
+/// if it is *not* nullptr, it will be set to the size of buffer that was
+/// allocated.  This is not necessarily the length of the string (it may be
+/// somewhat higher).
+///
+/// Otherwise, the result will be written into the output buffer, and the
+/// size of the result will be written into outputBufferSize.  If the buffer
+/// is too small, the result will be truncated, but outputBufferSize will
+/// still be set to the number of bytes that would have been required to
+/// copy out the full result (including a trailing NUL).
+///
+/// The unusual behaviour here is a consequence of the way the C++ ABI's
+/// demangling function works.
+///
+/// @returns a pointer to the output if demangling was successful.
 SWIFT_RUNTIME_STDLIB_SPI
 char *_swift_backtrace_demangle(const char *mangledName,
                                 size_t mangledNameLength,

stdlib/public/runtime/Backtrace.cpp

@@ -850,33 +850,7 @@ _swift_isThunkFunction(const char *mangledName) {
   return ctx.isThunkSymbol(mangledName);
 }
 
-/// Try to demangle a symbol.
-///
-/// Unlike other entry points that do this, we try both Swift and C++ here.
-///
-/// @param mangledName is the symbol name to be demangled.
-/// @param mangledNameLength is the length of this name.
-/// @param outputBuffer is a pointer to a buffer in which to place the result.
-/// @param outputBufferSize points to a variable that contains the size of the
-/// output buffer.
-/// @param status returns the status codes defined in the C++ ABI.
-///
-/// If outputBuffer is nullptr, the function will allocate memory for the
-/// result using malloc().  In this case, outputBufferSize may be nullptr;
-/// if it is *not* nullptr, it will be set to the size of buffer that was
-/// allocated.  This is not necessarily the length of the string (it may be
-/// somewhat higher).
-///
-/// Otherwise, the result will be written into the output buffer, and the
-/// size of the result will be written into outputBufferSize.  If the buffer
-/// is too small, the result will be truncated, but outputBufferSize will
-/// still be set to the number of bytes that would have been required to
-/// copy out the full result (including a trailing NUL).
-///
-/// The unusual behaviour here is a consequence of the way the C++ ABI's
-/// demangling function works.
-///
-/// @returns a pointer to the output if demangling was successful.
+// Try to demangle a symbol.
 SWIFT_RUNTIME_STDLIB_SPI char *
 _swift_backtrace_demangle(const char *mangledName,
                           size_t mangledNameLength,