Allow ELF section flags to be specified numerically

[prakhar]

Summary

GAS already allows flags for sections to be specified directly as a
numeric value. This functionality is particularly useful for setting
processor or application-specific values that may not be directly
supported or understood by LLVM. This patch allows LLVM to use numeric
section flag values verbatim if specified by the assembly file.

[rengolin]

LGTM. This is in accordance with the document:

https://sourceware.org/binutils/docs-2.27/as/Section.html#Section

llvm/trunk/lib/MC/MCParser/ELFAsmParser.cpp

if (Size == 0)
     return true;
 
   return false;
 }
 
 static unsigned parseSectionFlags(StringRef flagsStr, bool *UseLastGroup) {
   unsigned flags = 0;
 
+  // If a valid numerical value is set for the section flag, use it verbatim
+  if (!flagsStr.getAsInteger(0, flags))
+    return flags;
+
   for (char i : flagsStr) {
     switch (i) {
     case 'a':
       flags |= ELF::SHF_ALLOC;
       break;
     case 'e':
       flags |= ELF::SHF_EXCLUDE;
       break;

llvm/trunk/test/MC/ELF/section-numeric-flag.s

+// RUN: llvm-mc -filetype=obj %s -o - \
+// RUN: | llvm-readobj -s -t | FileCheck %s
+
+        .section .text,    "0x806", %progbits, unique, 0
+        .section .comment, "0x21"
+
+
+// CHECK:      Section {
+// CHECK:        Name: .text (1)
+// CHECK-NEXT:   Type: SHT_PROGBITS (0x1)
+// CHECK-NEXT:   Flags [ (0x6)
+// CHECK-NEXT:     SHF_ALLOC (0x2)
+// CHECK-NEXT:     SHF_EXECINSTR (0x4)
+// CHECK-NEXT:   ]
+// CHECK:        Size: 0
+// CHECK:      }
+
+// CHECK:      Section {
+// CHECK:        Name: .text (1)
+// CHECK-NEXT:   Type: SHT_PROGBITS (0x1)
+// CHECK-NEXT:   Flags [ (0x806)
+// CHECK-NEXT:     SHF_ALLOC (0x2)
+// CHECK-NEXT:     SHF_COMPRESSED (0x800)
+// CHECK-NEXT:     SHF_EXECINSTR (0x4)
+// CHECK-NEXT:   ]
+// CHECK:        Size: 0
+// CHECK:      }
+
+// CHECK:      Section {
+// CHECK:        Name: .comment (7)
+// CHECK-NEXT:   Type: SHT_PROGBITS (0x1)
+// CHECK-NEXT:   Flags [ (0x21)
+// CHECK-NEXT:     SHF_STRINGS (0x20)
+// CHECK-NEXT:     SHF_WRITE (0x1)
+// CHECK-NEXT:   ]
+// CHECK:        Size: 0
+// CHECK:      }