storage, db, mixer, command: remove more bogus "pure" attributes

This commit is similar to 788e3b31e1,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

NEWS

@@ -2,6 +2,7 @@ ver 0.20.9 (not yet released)
 * decoder
   - ffmpeg: support *.adx
 * fix byte order detection on FreeBSD/aarch64
+* fix more random crashes when compiled with clang
 
 ver 0.20.8 (2017/05/19)
 * output

src/command/Request.hxx

@@ -45,68 +45,57 @@ public:
 			     : default_value;
 	}
 
-	gcc_pure
 	int ParseInt(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgInt(data[idx]);
 	}
 
-	gcc_pure
 	int ParseInt(unsigned idx, int min_value, int max_value) const {
 		assert(idx < size);
 		return ParseCommandArgInt(data[idx], min_value, max_value);
 	}
 
-	gcc_pure
 	int ParseUnsigned(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgUnsigned(data[idx]);
 	}
 
-	gcc_pure
 	int ParseUnsigned(unsigned idx, unsigned max_value) const {
 		assert(idx < size);
 		return ParseCommandArgUnsigned(data[idx], max_value);
 	}
 
-	gcc_pure
 	bool ParseBool(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgBool(data[idx]);
 	}
 
-	gcc_pure
 	RangeArg ParseRange(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgRange(data[idx]);
 	}
 
-	gcc_pure
 	float ParseFloat(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgFloat(data[idx]);
 	}
 
-	gcc_pure
 	SongTime ParseSongTime(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgSongTime(data[idx]);
 	}
 
-	gcc_pure
 	SignedSongTime ParseSignedSongTime(unsigned idx) const {
 		assert(idx < size);
 		return ParseCommandArgSignedSongTime(data[idx]);
 	}
 
-	gcc_pure
 	int ParseOptional(unsigned idx, int default_value) const {
 		return idx < size
 			? ParseInt(idx)
 			: default_value;
 	}
 
-	gcc_pure
 	RangeArg ParseOptional(unsigned idx, RangeArg default_value) const {
 		return idx < size
 			? ParseRange(idx)

src/db/Interface.hxx

@@ -106,7 +106,6 @@ public:
 				     TagType tag_type, tag_mask_t group_mask,
 				     VisitTag visit_tag) const = 0;
 
-	gcc_pure
 	virtual DatabaseStats GetStats(const DatabaseSelection &selection) const = 0;
 
 	/**

src/mixer/MixerInternal.hxx

@@ -84,7 +84,6 @@ public:
 	 * @return the current volume (0..100 including) or -1 if
 	 * unavailable
 	 */
-	gcc_pure
 	virtual int GetVolume() = 0;
 
 	/**

src/pcm/Resampler.hxx

@@ -63,11 +63,12 @@ public:
 	/**
 	 * Resamples a block of PCM data.
 	 *
+	 * Throws std::runtime_error on error.
+	 *
 	 * @param src the input buffer
 	 * @return the destination buffer (will be invalidated by
 	 * filter_close() or filter_filter())
 	 */
-	gcc_pure
 	virtual ConstBuffer<void> Resample(ConstBuffer<void> src) = 0;
 };
 

src/storage/StorageInterface.hxx

@@ -39,7 +39,6 @@ public:
 	/**
 	 * Throws #std::runtime_error on error.
 	 */
-	gcc_pure
 	virtual StorageFileInfo GetInfo(bool follow) = 0;
 };
 
@@ -52,7 +51,6 @@ public:
 	/**
 	 * Throws #std::runtime_error on error.
 	 */
-	gcc_pure
 	virtual StorageFileInfo GetInfo(const char *uri_utf8, bool follow) = 0;
 
 	/**