Add documentation and tests for _Pragma.

Tests/Conformance/DOIT3

@@ -22,6 +22,7 @@
 {1} c99math.c
 {1} c99fpcmp.c
 {1} c99tgmath.c
+{1} c99pragma.c
 {1} c11generic.c
 {1} c11align.c
 {1} c11noret.c

Tests/Conformance/c99pragma.c

@@ -0,0 +1,23 @@
+/*
+ * Test _Pragma preprocessing operator (C99).
+ */
+
+#include <stdio.h>
+
+#pragma optimize -1
+#pragma debug -1
+
+_Pragma("debug 0")
+
+#define p(x) _Pragma(#x)
+#define opt(x) p(optimize x)
+
+void f(int n, ...) {
+}
+
+opt(1+2)
+
+int _Pragma("")main(_Pragma("*#^abcdefg foo bar baz")void) {
+        f(123, 456, 789);
+        printf _Pragma("unrecognized")("Passed Conformance Test c99pragma\n");
+}

cc.notes

@@ -583,6 +583,20 @@ Members of anonymous structures or unions (including nested ones) may be accesse
 
 The _Thread_local storage-class specifier may be used together with static or extern.  This has the normal effect of the other storage-class specifier while also indicating that the variable has thread-local storage.  For declarations within a function, _Thread_local must be used together with static or extern.
 
+29. (C99) ORCA/C now supports the _Pragma preprocessing operator.  A token sequence of the form
+
+   _Pragma ( string-literal )
+
+is processed as if the contents of the string literal were the operands of a #pragma preprocessing directive.  For example,
+
+   _Pragma("optimize 1")
+
+is equivalent to the directive
+
+   #pragma optimize 1
+
+The _Pragma("...") token sequence may be formed using preprocessor macros.
+
 
 Multi-Character Character Constants
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