[NFC][builtins] Make softfloat-related errors less noisy

[atrosinenko]

Summary

Choose SINGLE_PRECISION by default in fp_lib.h after manually #erroring when no precision is set. This eliminates cluttering log with lots of errors like "unknown type name 'fp_t'" after the descriptive error message was already generated manually. In this patch, I assume that #error cannot be silenced.

This makes clang-tidy output during differential review of patches more relevant for fp_*_impl.inc while checking macroses even stricter (checking that *exactly* one is defined instead of *at least* one is).

Example output of clang-tidy for fp_add_impl.inc:

fp_add_impl_inc-tidy.before.txt9 KBDownload

fp_add_impl_inc-tidy.after.txt6 KBDownload

[atrosinenko]

Rebase the entire patch stack against the up-to-date master and re-upload.

compiler-rt/lib/builtins/fp_lib.h

 #define FP_LIB_HEADER
 
 #include "int_lib.h"
 #include "int_math.h"
 #include <limits.h>
 #include <stdbool.h>
 #include <stdint.h>
 
+#if (defined(SINGLE_PRECISION) + defined(DOUBLE_PRECISION) +                   \
+     defined(QUAD_PRECISION)) != 1
+#error Exactly one of SINGLE_PRECISION, DOUBLE_PRECISION or QUAD_PRECISION must be defined.
+// Now, turn on some arbitrary one of the three precision settings (except for
+// QUAD_PRECISION that has some specific requirements), so fp_*_impl.inc files
+// could be validated by clang-tidy in a more meaningful way.
+#define SINGLE_PRECISION
+#endif
+
 // x86_64 FreeBSD prior v9.3 define fixed-width types incorrectly in
 // 32-bit mode.
 #if defined(__FreeBSD__) && defined(__i386__)
 #include <sys/param.h>
 #if __FreeBSD_version < 903000 // v9.3
 #define uint64_t unsigned long long
 #define int64_t long long
 #undef UINT64_C

 #undef Word_2
 #undef Word_3
 #undef Word_4
 #undef Word_HiMask
 #undef Word_LoMask
 #undef Word_FullMask
 #endif // __LDBL_MANT_DIG__ == 113 && __SIZEOF_INT128__
 #else
+// Should never be reached due to the check for precison-related macro
+// at the top of this file. Recheck it here just in case...
 #error SINGLE_PRECISION, DOUBLE_PRECISION or QUAD_PRECISION must be defined.
 #endif
 
 #if defined(SINGLE_PRECISION) || defined(DOUBLE_PRECISION) ||                  \
     defined(CRT_LDBL_128BIT)
 #define typeWidth (sizeof(rep_t) * CHAR_BIT)
 #define exponentBits (typeWidth - significandBits - 1)
 #define maxExponent ((1 << exponentBits) - 1)