[C] Add support for constexpr storage class specifier (#4071)

This was added in C23 but is more limited in scope than the C++
version. The C version can only be applied to variables to indicate
their usability as a compile time constant.

C++/C.sublime-syntax

@@ -44,7 +44,7 @@ variables:
   windows_types: 'APIENTRY|ATOM|BOOL|BOOLEAN|BYTE|CALLBACK|CCHAR|CHAR|COLORREF|CONST|DWORD|DWORDLONG|DWORD_PTR|DWORD32|DWORD64|FLOAT|HACCEL|HALF_PTR|HANDLE|HBITMAP|HBRUSH|HCOLORSPACE|HCONV|HCONVLIST|HCURSOR|HDC|HDDEDATA|HDESK|HDROP|HDWP|HENHMETAFILE|HFILE|HFONT|HGDIOBJ|HGLOBAL|HHOOK|HICON|HINSTANCE|HKEY|HKL|HLOCAL|HMENU|HMETAFILE|HMODULE|HMONITOR|HPALETTE|HPEN|HRESULT|HRGN|HRSRC|HSZ|HWINSTA|HWND|INT|INT_PTR|INT8|INT16|INT32|INT64|LANGID|LCID|LCTYPE|LGRPID|LONG|LONGLONG|LONG_PTR|LONG32|LONG64|LPARAM|LPBOOL|LPBYTE|LPCOLORREF|LPCSTR|LPCTSTR|LPCVOID|LPCWSTR|LPDWORD|LPHANDLE|LPINT|LPLONG|LPSTR|LPTSTR|LPVOID|LPWORD|LPWSTR|LRESULT|PBOOL|PBOOLEAN|PBYTE|PCHAR|PCSTR|PCTSTR|PCWSTR|PDWORD|PDWORDLONG|PDWORD_PTR|PDWORD32|PDWORD64|PFLOAT|PHALF_PTR|PHANDLE|PHKEY|PINT|PINT_PTR|PINT8|PINT16|PINT32|PINT64|PLCID|PLONG|PLONGLONG|PLONG_PTR|PLONG32|PLONG64|POINTER_32|POINTER_64|POINTER_SIGNED|POINTER_UNSIGNED|PSHORT|PSIZE_T|PSSIZE_T|PSTR|PTBYTE|PTCHAR|PTSTR|PUCHAR|PUHALF_PTR|PUINT|PUINT_PTR|PUINT8|PUINT16|PUINT32|PUINT64|PULONG|PULONGLONG|PULONG_PTR|PULONG32|PULONG64|PUSHORT|PVOID|PWCHAR|PWORD|PWSTR|QWORD|SC_HANDLE|SC_LOCK|SERVICE_STATUS_HANDLE|SHORT|SIZE_T|SSIZE_T|TBYTE|TCHAR|UCHAR|UHALF_PTR|UINT|UINT_PTR|UINT8|UINT16|UINT32|UINT64|ULONG|ULONGLONG|ULONG_PTR|ULONG32|ULONG64|UNICODE_STRING|USHORT|USN|VOID|WCHAR|WINAPI|WORD|WPARAM'
   stdint: 'int8_t|int16_t|int32_t|int64_t|uint8_t|uint16_t|uint32_t|uint64_t|int_least8_t|int_least16_t|int_least32_t|int_least64_t|uint_least8_t|uint_least16_t|uint_least32_t|uint_least64_t|int_fast8_t|int_fast16_t|int_fast32_t|int_fast64_t|uint_fast8_t|uint_fast16_t|uint_fast32_t|uint_fast64_t|intptr_t|uintptr_t|intmax_t|intmax_t|uintmax_t|uintmax_t'
   declspec: '__declspec\(\s*\w+(?:\([^)]+\))?\s*\)'
-  storage_classes: 'static|extern|register|thread_local|_Thread_local|{{declspec}}'
+  storage_classes: 'static|extern|register|thread_local|_Thread_local|constexpr|{{declspec}}'
   type_qualifier: 'const|volatile'
   compiler_directive: 'inline|restrict|__restrict__|__restrict'
   modifiers: '{{storage_classes}}|{{type_qualifier}}|{{compiler_directive}}'

C++/syntax_test_c.c

@@ -734,6 +734,9 @@ static const uint32_t * const MACRO funcname();
 /*                      ^ storage.modifier */
 /*                                  ^ entity.name.function */
 
+static constexpr int bar = 1;
+/*     ^ storage.modifier */
+
 MACRO int
 /*    ^ storage.type */
 funcname2