pregenerate: Compile public headers entirely as C.

After I43281918d498a0c78292d12c01a6df5233938228, we essentially already
are doing that - however let's also tell the compiler to slightly
simplify things a bit more.

The only catch is that in C mode, functions become `static inline` in C
when they'd be `inline` (and thus get weak linker symbols) in C++.

Simplifies compilation a bit more (on my system down from 6.40s to
6.30s user time), but the primary win is consistency.

Note that in case this ever misses a symbol, audit-symbols.go being run
on CQ will flag it. As a corollary, that this change passes CQ without
`prefix-symbols.h` changing is evidence for its correctness.

Change-Id: If23753cb482268466ee52ce3d39665b66a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/94127
Commit-Queue: David Benjamin <davidben@google.com>
Auto-Submit: Rudolf Polzer <rpolzer@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Benjamin <davidben@google.com>

util/pregenerate/idextractor.go

diff --git a/util/pregenerate/idextractor.go b/util/pregenerate/idextractor.go
index a564b27..8c6b2db 100644
--- a/util/pregenerate/idextractor.go
+++ b/util/pregenerate/idextractor.go
@@ -82,27 +82,25 @@
 	if isCL {
 		// If using clang-cl.exe, args need to be in CL form.
 		args = []string{
-			"/TP",
-			"/std:c++17",
-			"/Zs",
-			"-Xclang", "-ast-dump=json",
-			"/I", "include",
 			// Suppress the C++ helper APIs, to avoid including STL headers. There are
 			// no C symbols in there, and this reduces the volume of JSON from around
 			// 288 MB to 44 MB.
-			"/DBORINGSSL_NO_CXX=1",
+			"/TC",
+			"/std:c99",
+			"/Zs",
+			"-Xclang", "-ast-dump=json",
+			"/I", "include",
 			"-",
 		}
 	} else {
 		// Standard Clang args.
 		args = []string{
-			"-x", "c++",
-			"-std=c++17",
+			// See above.
+			"-x", "c",
+			"-std=c99",
 			"-fsyntax-only",
 			"-Xclang", "-ast-dump=json",
 			"-Iinclude",
-			// See above.
-			"-DBORINGSSL_NO_CXX=1",
 			"-",
 		}
 	}
@@ -150,12 +148,13 @@
 		}
 		var isInline bool
 		switch id.Linkage {
-		case "", "static", "static inline":
+		case "", "static":
 			// Definitely not linked.
 			return nil
-		case `extern "C" inline`, `extern "C++" inline`:
-			// Sorry, can't redefine_extname inline functions:
+		case `extern "C" inline`, `extern "C++" inline`, "static inline":
+			// Sorry, can't redefine_extname inline functions in GCC:
 			// error: #pragma redefine_extname is applicable to external C declarations only; not applied to function
+			// Also, including `static inline` as it becomes `inline` when compiling as C++.
 			isInline = true
 		case `extern "C"`:
 			// Link those.
@@ -189,7 +188,7 @@
 		}
 	}
 
-	err = idextractor.New(report, idextractor.Options{Language: "C++"}).Parse(stdout)
+	err = idextractor.New(report, idextractor.Options{Language: "C"}).Parse(stdout)
 	if err != nil {
 		c.Process.Kill()
 		return cSymbolData{}, err