[ARM,AArch64] Store source location of asm constant pool entries

[ARM,AArch64] Store source location of asm constant pool entries

Storing the source location of the expression that created a constant pool
entry allows us to emit better error messages if we later discover that the
expression cannot be represented by a relocation.

Differential Revision: http://reviews.llvm.org/D14646

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253220 91177308-0d34-0410-b5e6-96231b3b80d8